malapropism \mal-uh-PROP-iz-uhm\ (noun) - The usually unintentionally humorous misuse of a word, especially by confusion with one of similar sound; also, an example of such misuse.
"The old gentleman, too, had publicly declared that the Fieldhead estate and the De Walden estate were delightfully contagious - a malapropism which rumour had not failed to repeat to Shirley." -- Charlotte Bronte, 'Shirley'
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A malapropism is so called after Mrs. Malaprop, a character noted for her amusing misuse of words in Richard Brinsley Sheridan's comedy The Rivals.