A malapropism (also called a Dogberryism or acyrologia) is the substitution of a word for a word with a similar sound, in which the resulting phrase makes no sense but often creates a comic effect. It is not the same as an eggcorn, which is a similar substitution in which the new phrase makes sense on some level. Occasionally a phrase, rather than a single word, replaces the original word, for example Stan Laurel said "What a terrible cat's after me!" (i.e., catastrophe) in Any Old Port! Click to advance slides
The word malapropos is an adjective or adverb meaning "inappropriate" or "inappropriately", derived from the French phrase mal à propos (literally "ill-suited"). The earliest English usage of the word cited in the Oxford English Dictionary is from 1630. Malaprop used in the linguistic sense was first used by LordByron in 1814 according to the OED. The terms malapropism and the earlier variant malaprop come from Richard Brinsley Sheridan's 1775 play The Rivals, and in particular the character Mrs. Malaprop. Sheridan presumably named his character Mrs. Malaprop, who frequently misspoke (to great comic effect), in joking reference to the word malapropos.
Here are some of the original malapropisms from the lady herself: Mrs. Malaprop in Richard Sheridan's play The Rivals (1775). In case you're not sure what it is that Mrs. Malaprop is intending to say, the correct word(s) are in square brackets after each quotation.
"...promise to forget this fellow - to illiterate him, I say, quite from your memory." [obliterate] "O, he will dissolve my mystery!" [resolve] "He is the very pine-apple of politeness!" [pinnacle] "I have since laid Sir Anthony's preposition before her;" [proposition]
"Oh! it gives me the hydrostatics to such a degree." [hysterics] "I hope you will represent her to the captain as an object not altogether illegible." [eligible] "...she might reprehend the true meaning of what she is saying." [comprehend] "...she's as headstrong as an allegory on the banks of the Nile." [alligator]
"I am sorry to say, Sir Anthony, that my affluence over my niece is very small." [influence] "Why, murder's the matter! slaughter's the matter! killing's the matter! - but he can tell you the perpendiculars." [particulars] "Nay, no delusions to the past - Lydia is convinced;" [allusions]
"I thought she had persisted from corresponding with him;" [desisted] "His physiognomy is so grammatical!" [phraseology] "I am sure I have done everything in my power since I exploded the affair;" [exposed] "I am sorry to say, she seems resolved to decline every particle that I enjoin her." [article]
"...if ever you betray what you are entrusted with... you forfeit my malevolence for ever..." [benevolence] "Your being Sir Anthony's son, captain, would itself be a sufficient accommodation;" [recommendation] "Sure, if I reprehend any thing in this world it is the use of my oracular tongue, and a nice derangement of epitaphs!" [apprehend, vernacular, arrangement, epithets]