I believe Anu gives a mistaken impression when he labels mumpsimus an eponym. It's not, since it is not derived from the name of a person, as is boycott. A proper description of mumpsimus is the mispronunciation of a word or phrase by the transposition of letters or syllables, and that's called a spoonerism, which is an eponym, after William Archibald Spooner, (1844-1930) a British cleric.