The term "folk etymology", as referring both to erroneous beliefs about derivation and the consequent changes to words, is derived from the German Volksetymologie. Similar terms are found in other languages, e.g. volksetymologie in Dutch, Afrikaans volksetimologie, Danish folkeetymologi, Swedish folketymologi, and full parallels in non-Germanic languages, e.g. Hungarian népetimológia, French étymologie populaire and Israeli Hebrew etimológya amamít (popular etymology). Examples of alternative names are Italian pseudoetimologia and paretimologia (<paraetimologia), as well as English etymythology.
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, sometimes right but more often wrong, based on superficial resemblances of form and the like. This popular etymology has had a powerful influence on the forms which words take (e.g. crawfish or crayfish, from the French crevis, modern crevisse, or sand-blind, from samblind, i.e. semi-, half-blind), and has frequently been the occasion of homonyms resulting from different etymologies for what appears a single word, with the original meaning(s) reflecting the true etymology and the new meaning(s) reflecting the 'incorrect' popular etymology.
More recent examples:
Other changes due to folk etymology include:
When a back-formation rests on a misunderstanding of the morphology of the original word, it may be regarded as a kind of folk etymology.
In heraldry, a rebus coat-of-arms (which expresses 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.
The same process sometimes influences the spelling of proper names. The name Antony/Anthony is often spelled with an "h" because of the Elizabethan belief that it is derived from Greek ανθος (flower). In fact it is a Roman family name, probably meaning something like "ancient".
The French verb savoir (to know) was formerly spelled sçavoir, in order to link it with the Latin scire (to know). In fact it is derived from sapere (to be wise).
The spelling of the English word posthumous reflects a belief that it is derived from Latin post humum, literally "after the earth", in other words after burial. In fact the Latin postumus is an old superlative of post (after), formed in the same way as optimus and ultimus.
The spelling of the English word lethal reflects a belief that it is derived from Lethe, the river in the mythological kingdom of the dead. In fact it comes from the unconnected Latin word letum, meaning death.
In British English, aubergines are sometimes called "mad apples". The Italian word for the aubergine is melanzana, which was misheard as mela insana.
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.
Olisipona (Lisbon) was explained as deriving from the city's supposed foundation by Ulysses (Odysseus), though the settlement certainly antedates any Greek presence.
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 ("well come" or "good wind") after the Roman conquest.
The Dutch word for "hammock" is hangmat, ("hanging mat") formed as a folk etymology of Spanish hamaca.
In the Alexandrian period, and in the Renaissance, many (wrongly) explained the name of the god Kronos as being derived from chronos (time), and interpreted the myth of his swallowing his children as an allegory meaning that Time consumes all things.
The American Grizzly bear is so named because its hair is grizzly or silver-tipped, but its name was later mistakenly derived from grisly meaning “horrible”. This error has been perpetuated in the grizzly bear's scientific trinomial name: Ursus arctos horribilis.