STFU is shorthand for Shut The Fuck Up.
In professional wrestling, it was also the name of John Cena's finishing submission hold. While this move is normally called an STF (Stepover Toehold Facelock), Cena's "Urban Thug" gimmick predicated that he come up with a name that is slightly more vile. The choice of name was also directed at his critics who claimed that he couldn't execute a proper wrestling hold. Previous to Cena's use of the move, it was rarely seen on US television, often reserved for only the most technical of wrestlers, such as Chris Benoit and Dean Malenko.