Folk etymology – also known as (generative) popular etymology, analogical reformation, (morphological) reanalysis and etymological reinterpretation – is a change in a word or phrase resulting from the replacement of an unfamiliar form by a more familiar one through popular usage. The form or the meaning of an archaic, foreign, or otherwise unfamiliar word is reinterpreted as resembling more familiar words or morphemes.

The term folk etymology is a loan translation from German Volksetymologie, coined by Ernst Förstemann in 1852. Folk etymology is a productive process in historical linguistics, language change, and social interaction. Reanalysis of a word's history or original form can affect its spelling, pronunciation, or meaning. This is frequently seen in relation to loanwords or words that have become archaic or obsolete.

Folk/popular etymology may also refer to a popular false belief about the etymology of a word or phrase that does not lead to a change in the form or meaning. To disambiguate the usage of the term "folk/popular etymology", Ghil'ad Zuckermann proposes a clear-cut distinction between the derivational-only popular etymology (DOPE) and the generative popular etymology (GPE): the DOPE refers to a popular false etymology involving no neologization, and the GPE refers to neologization generated by a popular false etymology.

Examples of words created or changed through folk etymology include the English dialectal form sparrowgrass, originally from Greek ἀσπάραγος ("asparagus") remade by analogy to the more familiar words sparrow and grass. When the alteration of an unfamiliar word is limited to a single person, it is known as an eggcorn.

The technical term "folk etymology" refers to a change in the form of a word caused by erroneous popular suppositions about its etymology. Until the academic development of comparative linguistics and description of laws underlying sound changes, the derivation of a word was mostly guess-work. Speculation about the original form of words in turn 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. This popular etymologizing has had a powerful influence on the forms which words take. Examples in English include crayfish or crawfish, which are not historically related to fish but come from Middle English crevis, cognate with French écrevisse. Likewise chaise lounge, from the original French chaise longue ("long chair"), has come to be associated with the word lounge.

Other types of language change caused by reanalysis of the structure of a word include rebracketing and back-formation.

In rebracketing, users of the language change, misinterpret, or reinterpret the location of a boundary between words or morphemes. For example, the Old French word orenge 'orange tree' comes from Arabic النَّرَنْج an-naranj 'the orange tree', with the initial ⟨n⟩ of naranj understood as part of the article. Rebracketing in the opposite direction saw the Middle English a napron become an apron.