A malapropism occurs when you say one word but you mean another, like instead of saying a certain restaurant is prosperous, you say it is preposterous. As you can tell, malapropisms are often humorous, though sometimes the joke is on the speaker.
The word malapropism, pronounced "mah-luh-PRAH-pih-zum," comes from the French phrase mal à propos, which means "ill-suited." Playwright Richard Brinsley Sheridan must have been thinking of the French phrase when he created his character Mrs. Malaprop, who made audiences howl with laughter when she used the wrong word. Examples include saying "allegory" instead of "alligator," and "illiterate him from your memory" instead of "obliterate."