Etymology of London
The etymology of the name of the city of London has been the subject of speculation for centuries, though no generally accepted explanation has been found. While there have been many theories advanced over the centuries, most can be dismissed as fanciful on linguistic or historical grounds. A few have been recognized as having some measure of academic plausibility, but none has any direct evidence.
The city of London was founded by the Romans in the 1st century CE though some earlier settlements are known to have existed. The roots Londin- and Lundin- are the most prevalent in names used from Roman times onward. The 12th-century account Historia Regum Britanniae asserts that the city's name is derived from the name of King Lud who once controlled the city. A variety of other theories have been proposed since.
Attested forms 
Richard Coates, in the 1998 article where he published his own theory of the etymology, lists all the known occurrences of the name up to around the year 900, in Greek, Latin, British and Anglo-Saxon.
Most of the older sources begin with Londin- (Λονδινιου, Londino, Londinium etc.), though there are some in Lundin-; but later examples are mostly Lundon- or London-, and all the Anglo-Saxon examples have Lunden- with various terminations. He observes that the modern spelling with <o> derives from a mediaeval writing habit of avoiding <u> between letters composed of minims.
Coates discusses various aspects of the phonemic form of the name, in order to be able to dismiss other suggestions and support his own proposal.
He asserts that "It is quite clear that these vowel letters in the earliest forms, both <o> and <u>, represent phonemically long vowel sounds": he refers to a number of other writers who have argued this, and adds several arguments of his own, including the form of the name in Welsh Llundein.
Coates discusses the ending of the name, whose exact shape he says is a problem. He observes that the ending in Latin sources before 600 is always -inium, which points to a British double termination -in-jo-n. But this cannot be the form from which the Anglo-Saxon names were borrowed, as they all have Lund-, and an /i/ in the following syllable would have caused Lynd- by umlaut. He tentatively accepts Jackson's argument that the British form was -on-jo-n, with the change to -inium unexplained. However he speculates further that the -i- could have arisen by metathesis of the -i- in the last syllable of his own suggested etymon (see below).
Proposed etymologies 
The earliest account of the toponym's derivation can be attributed to Geoffrey of Monmouth. In Historia Regum Britanniae, the name is described as originating from King Lud, who seized the city and ordered it to be renamed in his honour as Kaerlud. This was then eventually slurred into Karelundein and then London. However, Geoffrey's work contains many fanciful suppositions about place-name derivation and the suggestion has no basis in linguistics.
Other fanciful theories over the years have been:
- William Camden reportedly suggested that the name might come from Brythonic lhwn (modern Welsh Llwn) meaning "grove" and town. Thus, giving the origin as Lhwn Town, translating to "city in the grove".
- John Jackson, writing in the Gentleman's Magazine in 1792, challenges the Llyn din theory (see below) on geographical grounds, and suggests instead a derivation from Glynn din - presumably intended as 'valley city'.
- Some British Israelites claimed that the Anglo-Saxons, assumed to be descendants of the Tribe of Dan, named their settlement lan-dan, meaning "abode of Dan" in Hebrew.
- An unsigned article in The Cambro Briton for 1821 supports the suggestion of Luna din ('moon fortress'), and also mentions in passing the possibility of Llong din ('ship fortress').
- Several theories were discussed in the pages of Notes and Queries on 27 December 1851, including Luandun (supposedly "city of the moon", a reference to the temple of Diana supposed to have stood on the site of St Paul's Cathedral), and Lan Dian or Llan Dian ("temple of Diana"). Another correspondent dismissed these, and reiterated the common Llyn din theory.
- In The Cymry of '76 (1855), Alexander Jones says that the Welsh name derives from Llyn Dain, meaining 'pool of the Thames'.
- An 1887 Handbook for Travellers asserts that "The etymology of London is the same as that of Lincoln" (Latin Lindum).
- The general Henri Nicolas Frey, in his 1894 book "Annamites et extrême-occidentaux: recherches sur l'origine des langues.", emphasizes the similarity between the name of the city and the two vietnamese words lœun and dœun which can both mean: low, inferior, muddy.
- Edward P. Cheney, in his 1904 book A Short History of England (p. 18), attributes the origin of the name to dun: "Elevated and easily defensible spots were chosen [in pre-Roman times], earthworks thrown up, always in a circular form, and palisades placed upon these. Such a fortification was called a dun, and London and the names of many other places still preserve that termination in varying forms."
- A New Variorum Edition of Shakespeare (1918) mentions a variant on Geoffrey's suggestion being Lud's town, although refutes it saying that the origin of the name was most likely Saxon.
- Another suggestion, published in The Geographical Journal in 1899, is that the area of London was previously settled by Belgae who named their outposts after townships in Belgium. Some of these Belgic toponyms have been attributed to the namesake of London including Lime, Douvrend, and Londinières.
Coates says (p. 211) that "The earliest non-mythic speculation ... centred on the possibility of deriving London from Welsh Llyn din, supposedly 'lake fort' (? or 'fort lake'). But llyn derives from British *Lind-, which is incompatible with all the early attestations.
H. D'Arbois de Jubainville suggested in 1899 that the name meant Londino's fortress. But Coates argues that there is no such personal name recorded, and that D'Arbois' suggested etymology for it (from Celtic *londo-, 'fierce') would have a short vowel. Coates notes that this theory was repeated by linguistics up to the 1960s, and more recently still in less specialist works.
"The first of the scientific explanations" according to Coates (p. 212) was from Giovanni Alessio in 1951. He proposed a Ligurian rather than a Celtic origin, with a root *lond-/lont- meaning 'mud' or 'marsh'. Coates' major criticisms are that this does not have the required long vowel (an alternative form Alessio proposes, *lōna, has the long vowel, but lacks the required consonant), and that there is no evidence of Ligurian in Britain.
The other suggestion that Coates considers worthy of discussion was by Jean-Gabriel Gigot in 1974. In an article principally about St Martin de Londres in Hérault in France, Gigot tries to apply the Germanic root proposed for that name (*lohna) to the topography of London.
Coates' own theory is that the name derives from an Old European (pre-Celtic) name *Plowonida, from Indo-European roots *plew-, which underlies words in different languages meaning 'flow', 'swim' and 'boat'; and *nejd-, an element meaning 'flow', found in various river names around Europe.
His suggestion is that this name, meaning either 'boat river' or 'swimming river' was applied to the Thames where it becomes too wide to ford, in the vicinity of London. (He does admit that compound names are comparatively rare for rivers in the Indo-European area, but they are not unknown). The settlement on its banks would then be named from it, with the suffix -on-jon, in either Old European or Celtic times, giving *(p)lowonidonjon. Indo-European /p/ regularly disappears in Celtic, so this would have gone through *Lowonidonjon and either *Lōondonjon or *Lōnidonjon to *Lūndonjon and hence Lūndein or Lūndyn. An advantage of the form *Lōnidonjon is that it could account for Latin Londinium by metathesis to *Lōnodinjon.
- Coates, Richard (1998). "A new explanation of the name of London". Transactions of the Philological Society 96 (2): 203–229. doi:10.1111/1467-968X.00027.
- Jackson, Kenneth H. (1938). "Nennius and the 28 cities of Britain". Antiquity 12: 44–55.
- Legends of London's Origins
- Prickett, Frederick (1842). The history and antiquities of Highgate, Middlesex.
- Jackson, John (1792). "Conjecture on the Etymology of London". The Gentleman's Magazine (Longman, Hurst, Rees, Orme, and Brown).
- Gold, David L (1979). "English words of supposed Hebrew origin in George Crabb's English Synonymes". American Speech (Duke University Press) 51 (1): 61–64. doi:10.2307/454531. JSTOR 454531. More than one of
- "Etymology of 'London'". The Cambro Briton: 42–43. 1821.
- Notes and Queries. 1852.
- Jones, Alexander (1855). The Cymry of '76. New York: Sheldon, Lamport.
- Baedeker, Karl (1887). London and Its Environs: Handbook for Travellers. K. Baedeker.
- Henry, Frey (1894). Annamites et extrême-occidentaux: recherches sur l'origine des langues.. Hachette et Cie.
- Furness (ed), Horace Howard (1918). A New Variorum Edition of Shakespeare. J B Lipincott & co. ISBN 0-486-21187-8.
- The Geographical Journal. 1899.
- D'Arbois de Jubainville, H (1899). La Civilisation des Celtes et celle de l'épopée homérique (in fr). Paris: Albert Fontemoing.
- Alessio, Giovanni (1951). "L'origine du nom de Londres'". Actes et Mémoires du troisième congrès international de toponymie et d'anthroponymie. Louvain: Instituut voor naamkunde. pp. 223–224. Unknown parameter
- Gigot, Jean-Gabriel (1974). "Notes sur le toponyme "Londres" (Hérault)". Revue international d'onomastique 26: 284–292.