Folk etymology is urban legendry disguised as the true history of a word.

A well-known bit of folk etymology, for example, is about the F word. You've probably heard at some time or other that it was originally an acronym for "For Unlawful Carnal Knowledge".

Not true, but interesting, and therefore viral.

Usually folk etymologies are based on the sound of a word, or the erroneous decomposition of a word that wasn't compound to begin with. An example would be where people say 'history' is a compound of 'his' and 'story' (and not, as it actually is, inherited from Greek 'historia').

Sometimes it'll even reshape the word according to its whim: You could get sparrowgrass from asparagus. For people who think 'history' has 'his' in it, nothing prevents them from creating such abominations as 'herstory'.

Learn to listen for dubious word-histories, and check them against your dictionary, which most likely will know better.

Contrary to misconceptions, folk etymology should not be used to describe an urban legend behind a word or phrase's origin. (The word "history" does not come from "his story"; actually the word "story" comes from "history" (through Latin historia). But this is not folk etymology.)

Folk etymology, also known as popular etymology, in etymology, is when an unfamiliar word is altered through common usage to resemble a superficially similar word or phrase. The Oxford English Dictionary defines it as

the popular perversion of the form of words in order to render it apparently significant

For example, the source of the words "crayfish" and "crawfish" (same thing) comes from the Old High German word krebiz "crab". This became crevice in Old French, which became crevise in Middle English. People started to spell the second syllable as fish, since it was an aquatic animal (though not a fish). Today we call them crayfish or crawfish because of variations in Anglo-Norman pronunciation.

More examples:

• cucaracha (Spanish) -> cockroach
• curtal (French: "to dock an animal's tail") -> curtail (to cut short or reduce)
• hayo makamak (Chinook: "plenty to eat") -> high muck-a-muck or muckety-muck (an important and arrogant person)
• ribbesper (Low German: "pickled pork ribs roasted on a spit") -> sparerib (a cut of pork separated from the bacon strip)

Other languages do it too.

• sauerkraut (German: "sour cabbage") -> choucroute (French: "cabbage-cabbage")

For bad spellers, a word which comes by folk etymology is usually easy to spell - that's how its current spelling came about, because it was easier for people to understand.