Ow! Quit It is a catchphrase used by Bart when he is being hit by someone or something.
Its next appearance is "Homer vs. the Eighteenth Amendment" when Bart is pinched by multiple classmates for not wearing green on St. Patrick's Day.
The line is also heard in the video game, The Simpsons: Hit and Run. If Bart is zapped by the flying wasp cameras, he will occasionally say the line and coins will be lost.
Bart is heard saying the line in the episode "Treehouse of Horror XX". In the segment "Don't Have a Cow, Mankind", when Homer discovers Mr. Burns' body, you can hear the line in the backgrouns as Lisa repeatedly rams bart's head into a treebranch.