This is an onomatopoeia for laughter. Its origin is the Korean onomatopoeia ㅋㅋㅋ, in which ㅋ stands for the "k" sound, like in raspy, stifled laughter. (I'm not sure why it wound up transliterated as "kekeke" rather than just "kkk", though.) Incidentally, the same onomatopoeia also exists in Japanese, written as けけけ.
kekeke zergling rush
by broomnotyour May 17, 2010