A false etymology (pseudoetymology, paraetymology or paretymology), sometimes called folk etymology although this is also a technical term in linguistics, is a specious (plausible but false) belief about the origins of specific words, often originating in "common-sense" assumptions. Because of their specious "obviousness", many are widely held.
Such etymologies often have the feel of urban legends, and can be much more colorful than the typical etymologies found in dictionaries, often involving stories of unusual practices in particular subcultures (e.g. Oxford students from non-noble families being supposedly forced to write sine nobilitate by their name, soon abbreviated to s.nob., hence the word snob). Many recent examples are "backronyms" (acronyms made up to explain a term), as in "snob", and "posh" for "port outward, starboard homeward"; many other sourced examples are listed in the article on backronyms.
Source and influence of false etymologies 
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scholars in the early modern period began to produce more reliable results, but many of their hypotheses have been superseded. Even today, knowledge in the field advances so rapidly that many of the etymologies in contemporary dictionaries are outdated.
Association with urban legends 
Some etymologies are part of urban legends, and seem to respond to a general taste for the surprising, counterintuitive and even scandalous. One common example has to do with the phrase rule of thumb, meaning a rough measurement. An urban legend has it that the phrase refers to an old English law under which a man could legally beat his wife with a stick no thicker than his thumb (though no such law ever existed).
In the United States, many of these scandalous legends have had to do with racism and slavery. Common words such as picnic, buck, and crowbar have been alleged to stem from derogatory terms or racist practices. The "discovery" of these alleged etymologies is often believed by those who circulate them to draw attention to racist attitudes embedded in ordinary discourse. On one occasion, the use of the word niggardly led to the resignation of a U.S. public official because it sounded similar to the word nigger, despite the two words being unrelated etymologically.
See also 
- List of common false etymologies
- false cognate and false friends
- Chinese translation of crisis
- Folk etymology
- Johannes Goropius Becanus
- Medieval etymology
- Phono-semantic matching
- Pseudoscientific language comparison
- Slang dictionary
- World Wide Words etymology of "rule of thumb"
- Urban Legends reference pages on supposed etymology of picnic
- Urban Legends reference pages on supposed etymology of buck
- Urban Legends reference pages on supposed origin of crowbar
- Article on the etymology of the word niggardly