In pro wrestling, an STF is a move where you trap one of your opponent's legs with yours and pull their head back with your forearm. It stands for stepover toehold facelock.
A submission holds used by many wrestlers mainly used by John Cena. STF stands for Stepover Toehold Facelock. John Cena used to call it the STFU. In the gif below it shows how Cena does it.