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.