A derogatory term for a homosexual, made famous by Quentin Tarantino's Reservoir Dogs.
Mr. Blonde: Eddie, if I was a butt cowboy, I wouldn't even throw you to the posse.
Nice Guy Eddie: Of course not, you'd keep me for yourself, you sick bastard. Four years of fuckin' punks up the ass you'd appreciate a piece of prime rib when you see one.
It is a derogative term used to offend some one who is of homosexual nature or perceived to be of homosexual nature because of something they have just done.
Charles's Father: "Charles, are you looking at gay porn again?"
Charles: "No it was a pop-up"
Charles's Father: "yes you are you filthy Butt Cowboy!"