A shortcut for "Shut the Fuck Up".
It saves the effort of actually writing these words.
Used usually in Online-Multi Games, as Counter Strike, Enemy Territory, UT, etc.
It is used especially for shuting up ppl who use the mic for chat talks, or swearing.
But it is also useful in a argument, as you get annoyd from the other idiot.
The most common uses are:
"STFU you fucking n00b!"
"OMG STFU NABBBB"
"oh, stfu, you're an idiot."