STFU= Shut The Fuck Up
When you want someone to keep quiet but can't say it aloud because you are out in public or your parents or your boss is there.
My girlfriend was annoying everybody with her constant meaningless chatter. Finally, I turned to her and said, "Jean, STFU."
I was walking down the street when the dogs behind the fence started to bark. I yelled, "STFU, STFU."