TK is an acronym for "team killing" or "team killer," and refers to an individual who kills his own teammates. In first person shooter (FPS) video games, this is possible when friendly fire (FF) is enabled.
Team killers are either subversive moles or NOOBs. Even so, TKers are usually banneed. Except at conservapedia, where they are made sysops.
Other nerdy uses
TK is also used as an acronym for "team kill" or "total kill" in some roleplaying games. A TK happens when every member of an adventuring party is killed. A TK usually marks the end of an adventure.