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STFU is internet-speak for "Start Talking Faster Underdog" Or, it can mean "being silent with a curse going vertical" It is used primarily in crowded internet chatrooms, when someone is telling a long story but it is isn't being heard due to the other chatter. It indicates that the person should start talking faster, in order to drown out the other people's conversations.
In some (incorrect) circles, STFU can be translated to mean "Start Talking Faster Underdog!" The original manuscript, ess tee eff uuu, is open to interpretation.
[07:22] <wicu> This one time, I broke my arm, it hurt like hell.
[07:25] <wicu> I was walking on the sidewalk when this skat3r came by.
[07:26] <aqualock> urmomlol STFU YOUR MOM LOL
[07:29] <wicu> He thought he was so cool.
[07:30] <wicu> So I clotheslined him.
[07:31] <wicu> But I hit his helmet and broke my fsking arm!
[07:32] <aqualock> Dude, STFU. This is a My Little Pony chat room.
[07:33] <wicu> Sorry, hard to typ fasst wsiht one arm, I'l rty.
[07:33] <wicu> antuway, hhel laffed aht me, so kickked ijhm in the nuts
[07:33] <wicu> it was so f8unny, he bawled likke hewaz a little bvaby
[07:33] <wicu> it was totlaly wortth thhea pian
[07:34] <wicu> jahaheahe
[07:34] <wicu> there
[07:35] <aqualock> ZOMG! I wanted to hear it all before I had to go on my 1337 sp34k1n6 r4|\/|p463
[07:36] * aqualock quits eating balls *
It should be noted that using STFU wrongly would probably get you banned from the Internet so remember to STFU properly.
edit Other Usages
STFU is also often confused with ancient Japanese Christian (Yes! They Exist!) Saint Fu (St Fu for short). However, this was solved by historical documents proving that Fu's real name was, in fact, St Fuck.