Definition: The usually unintentionally humorous misuse or distortion of a word or phrase; especially: the use of a word sounding somewhat like the one intended but ludicrously wrong in the context.
Etymology: Mrs. Malaprop, character noted for her misuse of words in R. B. Sheridan's comedy The Rivals (1775) - (Miriam-Webster’s 11th Collegiate Dictionary).
"I don't want anybody stepping on anyone else's thunder."
"You can't pull the sheep over my eyes!"
"I don't mean to take the steam out of your sails, but...."
"I used to be as sharp as a button."
"That way I can kill two bricks with one stone."
"If my grandfather was alive today, he'd be turning in his grave."
"You can't teach an old leopard how to change its spots."
*Thanks to Pastor Tim for this joke!* http://www.cybersalt.org/cleanlaugh
[Crosswalk You Make Me Laugh – August 21, 2008]