Folk etymology is change in a word or phrase over time resulting from the replacement of an unfamiliar form by a more familiar one. Unanalyzable borrowings from foreign languages, like asparagus, or old compounds such as samblind which have lost their iconic motivation (since one or more of the morphemes making them up, like sam-, which meant "semi-", has become obscure) are reanalyzed in a more or less semantically plausible way, yielding, in these examples, sparrow grass and sandblind.
The term folk etymology, a loan translation from the 19th-century academic German Volksetymologie, is a technical one in philology and historical linguistics, referring to the change of form in the word itself, not to any actual explicit popular analysis.
As a productive force 
The technical term "folk etymology", a translation of the German Volksetymologie from Ernst Förstemann's essay Ueber Deutsche Volksetymologie in the 1852 work Zeitschrift für vergleichende Sprachforschung auf dem Gebiete des Deutschen, Griechischen und Lateinischen (Journal of Comparative Linguistic Research in the Areas of German, Greek and Latin), is used in the science of historical linguistics to refer to a change in the form of a word caused by erroneous popular beliefs about its derivation.
Erroneous etymologies can exist for many reasons. Some are reasonable interpretations of the evidence that happen to be false. For a given word there may often have been many serious attempts by scholars to propose etymologies based on the best information available at the time, and these can be later modified or rejected as linguistic scholarship advances. The results of medieval etymology, for example, were plausible given the insights available at the time, but have mostly been rejected by modern linguists. The etymologies of humanist scholars in the early modern period began to produce more reliable results, but many of their hypotheses have been superseded. Until academic linguistics developed the comparative study of philology and the development of the laws underlying sound changes, the derivation of words was a matter mostly of guess-work.
The phenomenon becomes especially interesting when it feeds back into the development of the word and thus becomes a part of a new etymology. Believing a word to have a certain origin, people begin to pronounce, spell, or otherwise use the word in a manner appropriate to that perceived origin, in a kind of misplaced pedantry. Thus a new standard form of the word appears which has been influenced by the misconception. This popular etymologizing has had a powerful influence on the forms which words take. Examples in English include "crayfish" or "crawfish", from the French crevis; "sand-blind", from the older samblind (i.e. semi-, half-blind); or "chaise lounge" for the original French chaise longue.
In heraldry, canting arms (which may express a name by one or more elements only significant by virtue of the supposed etymology) may reinforce a folk etymology for a noun proper, usually of a place.
Examples of words modified by folk etymology 
In linguistic change caused by folk etymology, the form of a word changes so that it better matches its popular rationalisation. Typically this happens either to unanalyzable foreign words or to compounds where the word underlying one part of the compound becomes obsolete.
Examples of Type A (foreign words):
- andiron, from Middle English aundyre, aundiren, was altered from Anglo-Norman andier by association with iron (ME ire, iren).
- causeway was modified from obsolete causey (French causée) to assimilate it with way.
- Charterhouse from Chartreuse, the feminine of Chartreux.
- cockroach was borrowed from Spanish cucaracha but was folk-etymologized as cock + roach.
- crayfish from Middle English crevis (from Anglo-Norman creveis), due to assimilation with fish.
- female (Old French femelle, diminutive of femme "woman"), by assimilation with male (Old French masle, from Latin masculus).
- liquorice, a British variant spelling of licorice, from the supposition that it has something to do with liquid, a supposition made twice before in Anglo-Normand licoris (influenced by licor "liquor") and Late Latin liquirītia (influenced by Latin liquēre), though the ultimate origin is Greek glykýrriza "sweet root".
- penthouse from pentice, borrowed from Anglo-Norman pentiz "attached building" (ultimately from Latin appendicium "appendage"). Note that pentice continues as a technical term in English.
- posthumous, as though related to humus, [grave-]soil, although it is a specialized sense of Latin postumus, "last [legitimate child]" i.e., one born after the death of the father.
- sparrow-grass, a dialectal form of asparagus.
- York, which came from the Old Norse Jórvík, meaning "horse bay", was re-interpreted from Old English Eoforwic, itself folk-etymologized as "wild-boar town". Eoforwic came from Latin Eboracum, borrowed from Celtic *Eborakon (cf. Welsh Efrog), meaning "yew thicket, stand of yew-trees" (cf. Scottish Gaelic iubhar, Welsh efwr "cow parsnip").
Examples of Type B (one part becomes obsolete):
- bridegroom from Old English bryd-guma "bride-man", after the Old English word guma "man" (cognate with Latin homo) fell out of use.
- the verb buttonhole in the sense "to detain in conversation", from buttonhold (originally a loop of string that held a button down)
- catty-corner and kitty-corner, modified from cater-corner, after cater "four" had become obsolete.
- curry favor from Middle English curry favel, after favel "chestnut horse" (a traditional symbol of duplicity) became obsolete.
- hangnail from Middle English agnail (Old English angnægl, cognate with anguish and anger).
- island was respelled from iland (although without any pronunciation change), from Old English ī(e)gland after ī(e)g "island" became obsolete. The new spelling was evidently based on an analysis of island as isle-land, from isle (an Old French word, going back to Latin insula).
- The archaic term lanthorn was a folk etymology from lantern (as old lanterns were glazed with strips of cows' horn), which never displaced the original term.
- sand-blind (as if "blinded by the sand") from Old English sam-blind "half-blind" (sam- is a once-common prefix cognate with "semi-").
- shamefaced from shamefast "caught in shame". In this case, the original meaning of fast — "fixed in place" — is not completely obsolete, but is restricted mostly to frozen expressions such as "stuck fast".
- wormwood replaced Middle English wermode, from Old English wermōd, with worm referring to its leaves being used as a vermifuge, and wood for its bitter taste; cf. dialectal German Wurmtod (< Wurm "worm") vs. standard Wermut or Dutch wormmoedt vs. wermoet. The Germanic terms (incl. Dutch wermoet) come from *warja-mōdō, a compound of warjanan "to hinder" + mōdaz "the mind", perhaps in reference to the effects of absinthism.
Examples of folk etymologies borrowed from other languages 
- A common incorrect explanation of the origin of the term windjammer consists of an introduction into English of a folk etymology of the term common in German and Dutch. Both these languages have a word similar to "jam" meaning "to wail" and since people were not aware that the term "windjammer" originally came from English, the folk etymology claims "windjammer" refers to the typical sound of strong winds blowing through the rigging. In fact, the word comes from the English word "to jam" because the sails are so large that they seem to "jam" the wind.
Examples of word meanings modified by a folk-etymology-like process 
A process similar to folk etymology may result in a change to the meaning of a word based on an imagined etymology connecting it to an unrelated but similar-sounding word. Often this comes about either through the confusion of a foreign or obsolete word (similar to types A and B above) with a more common word, but it can also result from confusion of two words that have become homophones. Examples:
- The term forlorn hope originally meant 'storming party, body of skirmishers' and is a borrowing from Dutch verloren hoop 'lost troop', where hoop is cognate with English heap. But confusion with English hope has given the term an additional meaning of 'hopeless venture'.
- council, which originally just meant a 'meeting' or 'group of people', but now means 'a committee that leads or governs' through confusion with the homophone counsel 'to give advice'.
- A jubilee was originally an ancient Jewish tradition (from the Hebrew word yovel, יובל), specifically a year of rest observed once every 50 years. However, it now often means a celebration or time of rejoicing (almost the opposite of its original meaning) through confusion with the unrelated word jubilant.
Further examples 
See the following articles that discuss folk etymologies for their subjects:
- belfry (architecture)
- chaise longue
- ducking stool
- Jerusalem artichoke (from Italian, girasole)
- Jordan almonds (from French, jardin)
- serviceberry (Sorbus)
- Welsh rarebit
Other languages 
The French verb savoir "to know" was formerly spelled sçavoir on the false belief it was derived from Latin scire "to know". In fact it comes from sapere "to be wise".
The Italian word liocorno "unicorn" is a folk etymology, based on lione (mod. leone) "lion", of older lunicorno (13th century), itself due to the fusion of il "the" + unicorno. Similarly, the medieval byform alicorno (14th century) was from a similar fusion (al "to the" + liocorno).
Medieval Latin widerdonum (Old French guerdon) was an alteration, due to confusion with Latin donum "gift", of Old High German widarlōn "reward, pay-back".
Medieval Latin has a word, bachelarius (bachelor), of uncertain origin, referring to a junior knight, and by extension to the holder of a university degree inferior to master or doctor. This was later re-spelled baccalaureus to reflect a false derivation from bacca laurea "laurel berry", alluding to the possible laurel crown of a poet or conqueror.
In Southern Italy in the Greek period there was a city Maloeis (gen. Maloentos), meaning "fruitful". This was rendered in Latin as Maleventum, "ill come" or "ill wind", and renamed Beneventum, "welcome" or "good wind", after the Roman conquest.
The Finnish compound word for "jealous" mustasukkainen literally means "black-socked" (musta "black" and sukka "sock"). However, the word is a case of a misunderstood loan translation from Swedish svartsjuk "black-sick". The Finnish word sukka fit with a close phonological equivalent to the Swedish sjuk 
Acceptance of resulting forms 
When a word changes in form or meaning owing to folk etymology, there is typically resistance to the change on the part of those who are aware of the true etymology. Many words altered through folk etymology survive beyond such resistance however, to the point where they entirely replace the original form in the language. Chaise lounge and Welsh rarebit are still often disparaged, for example, but shamefaced and the verb buttonhole are universally accepted. See prescription and description.
See also 
- Chinese word for "crisis"
- Corruption (linguistics)
- Expressive loan
- False etymology
- Johannes Goropius Becanus
- Phono-semantic matching
- Pseudoscientific language comparison
- Slang dictionary
- Oxford English Dictionary Online, "folk-etymology, usually, the popular perversion of the form of words in order to render it apparently significant"
- Routledge Dictionary of Language and Linguistics Folk Etymology
- R.L. Trask, Dictionary of Historical and Comparative Linguistics, Folk Etymology
- "Folk Etymology", p 142, The Concise Oxford Dictionary of Linguistics
- "Folk Etymology" Winfred Lehmann, Historical linguistics: an Introduction.
- Sihler, Andrew L. (2000). Language History: an introduction. John Benjamins. pp. 86–88. ISBN 978-90-272-3697-5.
- Raimo Anttila, Historical and Comparative Linguistics (Benjamins, 1989) ISBN 90-272-3557-0, pp 92-93
- Ernst Förstemann's essay Ueber Deutsche Volksetymologie in the 1852 work Zeitschrift für vergleichende Sprachforschung auf dem Gebiete des Deutschen, Griechischen und Lateinischen
- "The Origins and Development of the English Language", 4th ed., Thomas Pyles and John Algeo, 1993.
- "The development of Late Latin liquiritia was in part influenced by Latin liquēre 'to flow', in reference to the process of treating the root to obtain its extract." Barnhart, Robert K. (1988). The Barnhart Dictionary of Etymology. H.W. Wilson. p. 593. ISBN 978-0-8242-0745-8.
- Longman Exams Dictionary CD
- Brown, Lesley (ed.). 2002. Shorter Oxford English Dictionary, vol. 1, A–M. 5th ed. Oxford: Oxford University Press, p. 1600.
- Anatoly Liberman (2005). Word Origins ... and How We Know Them: Etymology for Everyone. Oxford University Press. ISBN 978-0-19-516147-2.
- Adrian Room (1986). Dictionary of True Etymologies. Routledge & Kegan Paul. ISBN 0-7102-0340-3.
- David Wilton (2004). Word Myths: Debunking Linguistic Urban Legends. Oxford University Press. ISBN 0-19-517284-1.
- Ghil'ad Zuckermann (2003). Language Contact and Lexical Enrichment in Israeli Hebrew. Houndmills: Palgrave Macmillan. (Palgrave Studies in Language History and Language Change, Series editor: Charles Jones). ISBN 1-4039-1723-X.
- Folk etymologies (a collection of folk etymologies)
- Richard Lederer, Spook Etymology on the Internet
- Popular fallacies in the attribution of phrase origins
- EtymologyOnLine - both true and folk etymologies- here mainly examples of popular etymologies