Chris Potts has told me about a case in which a woman wrote "egg corns" for "acorns." This might be taken to be a folk etymology, like "Jerusalem" for "girasole" in "Jerusalem artichoke" (a kind of sunflower). But it might also be treated as something like a mondegreen (also here and here), the kind of "slip of the ear" that is especially common in learning songs and poems. Finally, it's also something like a malapropism, where a word is mistakenly substituted for one of similar sound shape.
Although the example is somewhat like each of these three named categories of errors, it's not exactly any of them. Can anyone suggest a better term?
At greater length:
It's not a folk etymology, because this is the usage of one person rather than an entire speech community.
It's not a malapropism, because "egg corn" and "acorn" are really homonyms (at least in casual pronunciation), while pairs like "allegory" for "alligator," "oracular" for "vernacular" and "fortuitous" for "fortunate" are merely similar in sound (and may also share some aspects of spelling and morphemic content).
It's not a mondegreen because the mis-construal is not part of a song or poem or similar performance.
Note, by the way, that the author of this mis-hearing may be a speaker of the dialect in which "beg" has the same vowel as the first syllable of "bagel". For these folks, "egg corn" and "acorn" are really homonyms, if the first is not spoken so as to artificially separate the words.
[update (9/30/2003): Geoff Pullum suggests that if no suitable term already exists for cases like this, we should call them "egg corns", in the metonymic tradition of "mondegreen", since the eponymous solution of "malapropism" and "spoonerism" is not appropriate.]Posted by Mark Liberman at September 23, 2003 12:33 PM