A person who engages in intentionally or unintentionally killing teammates in online games. TKs are either unskilled or are trying to ruin the gaming experience for others. The latter can be summed up by the Battlefield 1942 term smacktard.
i tH3 l337 Ov TK!!! looooool
somone is then at liberty to call a 'TK', and the job is done.
"And then i shagged your mum and she loved it!!"
Girl 2: Aww hes so cute!
Girl 1: I know right?
Girl 2: I love that kid.