Ow! Quit It is a catchphrase used by Bart when he is being hit by someone or something.
It's first heard in the very first episode, "Simpsons Roasting on an Open Fire". When Lisa, Homer, and Maggie are poking his arm where he got his tattoo surgically removed.
It was heard again in "Bart Gets Hit by a Car" when Dr. Hibbert touches his wound on his forehead and pokes him on his toe.
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 background as Lisa repeatedly rams Bart's head into a treebranch.