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Kill Stealing Fuck. Someone who either kill steal's habitually or kill steal's something that is worth a large amount of XP.
(Guy2 smashes into the dungeon you have been fighting though for hours and kills the boss)

Guy1: "Guy2 is such a ksf!"
by awesome gamr February 23, 2010
knee slap funny.
used like lol.
person 1: yesterday i fell and broke my nose
person 2: KSF!!
by faenface August 11, 2008