Green’s Dictionary of Slang

S.T.F.O.! excl.

[abbr. Step The Fuck Off!]

(US black) an excl. denoting that someone has said something offensive or behaved unacceptably.

[US]G. Tate ‘Public Enemy’ in Flyboy in the Buttermilk (1992) 127: Whoa! S.T.F.O.! Would you say that to your mother, motherfucker?