Folk etymology is a linguistic phenomenon whereby borrowed or archaic phrases are reinterpreted according to analogy with other comon words or phrases in the language. Etymology refers to the origin of words. For example, the etymology of etymology can be traced through Old English and Latin to the Greek roots etymo, meaning 'true', and logos, meaning 'word'. In folk etymology, speakers af a language assume the etymology of a word or phrase by comparing it to similar-sounding words or phrases that already exist in the language.
A word or phrase is typically considered a folk etymology only if it has changed from its original borrowed form as a result of the reinterpreted etymology. If speakers assume an "incorrect" origin of a word or phrase, but its pronunciation and/or spelling are unchanged, then the term is not referred to as a folk etymology. For instance, some people assume that the English word history is a combination of the words his and story, but the word actually can be traced through Old French and Latin to the Greek root historia, meaning 'knowledge through inquiry, record, or narrative'. While the interpretation "his story" is a folk etymology, the word history is not properly referred to as such, as the reinterpretation does not affect its form.
There are many examples of folk etymology in common English words and phrases. For example, penthouse is derived from the Old French apentiz, meaning roughly, 'that which is appended to', but English speakers reinterpreted the word to include the English house, since a penthouse is a place where someone lives. Similarly, primrose, a type of flower, was reinterpreted by way of folk etymology to include the English name of another flower, rose, although the word was originally borrowed from Old French primerole.
Words that are not borrowed can also be subject to folk etymology if the original derivation becomes obsolete. Modern English hangnail is said to derive from Old English agnail, not related to hanging or nails, but rather referring to a painful corn on the foot and derived from a Proto-Indo-European root meaning 'tight' or 'painful'. Bridegroom derives from Old English bryd, meaning 'bride', and guma, meaning 'man'. However, when guma disappeared from the language, the term was reinterpreted to include the more modern word groom.