Acronym for Stop Being A Bitch. Usually used when a person is annoying, obnoxious, whiny, bratty, and bitchy.
Jon: "Sorry, the car broke down. We'll have to walk to the movie theater."
Lisa: "Oh My God! I can't take anymore of your stupid car! Go fix, sell it, or do something if you ever want to tap this ass again!"
Jon: " STFU
! By the way you're ugly!"