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.