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:

1) Acronym used for the phrase "shut the fuck up" for efficiency reasons.

A way of saying "shut the fuck up", instead of actually typing it. Has less meaning than actually typing "shut the fuck up", and is often said by people that are losing an argument or can't think of a comeback.

Southern Tenant Farmers' Union (STFU): Large labor union, formed in Arkansas in 1934, which lobbied the federal government for better New Deal reforms. Dissolved in 1960. Commonly causes mirth among US History students who are aware of the acronym's double meaning.

1.) Save The Freaking Unicorns
2.) Say, Thanks For Understanding
3.) Short Term Follow Up
4.) Special Task Force Union
5.) Stop Talking Find Username
6.) Slippery Teflon From Utah
7.) Stinky Telnet Fell Under
8.) Special Transitory Food Units
9.) Stealing Turkeys From Uncle
10.) Superior Tactical Fragging Unit
11.) Silence That Followed Us
12.) South Texas Farming University
13.) Southern Tenant Farmers Union
14.) Scrawny Two Faced Underling
15.) Samurai Training Finished Unsuccessfully
16.) Show Them Fury Unleashed

Or the most common definition:
17.) Shut The Fuck Up
The common response to the previous 16 definitions.

Shut the fuck up. It's simple. When you are annoyed with someone who won't stop ranting on instant messaging, you type "stfu!!" But be sure to limit yourslef to two exclamation points, though, because three will look too angry. You could also type two exclamation points and then a "1". Like this: stfu!!1. This way, the third exclamation point is there, but only in spirit. This puts a little extra anger, but not so enough to make the other person angry at you. :)

Shut-the-fuck-up: a term used when a normal request for someone to be quiet seems to fall on deaf ears; may be used in either a normal or assertive tone of voice.

online text-speak for "shut the fuck up".