Pronounced gee kee soy kee, it stands for go kill yourself or i'll kill you.
Pretty much can only be used amongst a group of about 5 people in WA, but if used among them they will know what it means and respond accordingly.
fmff: dude, im so good i have a car n stuff
fatom: *somehow says* >.< *in words*
fated: man im a loser and fat
skill: seriously, all of you should gkysoiky.
rest of people: okay