kekw comes from the korean way to write laughter,ㅋㅋ(keke), it was transliterated into english as kek, and the W was added when it became i common twitch emote
Laughter. Deriving from the korean version of LOL (KEK), and adding a "W" to the end exaggerating it. It is widely used as a twitch emote or as a common expression when something funny happened, to convey laughter.
Gamer 1: "Dude, this Guy just killed himself in front of me ingame."
Gamer2. "kekw. What an Idiot."
Twitch emotes with the -W suffix often convey an exaggerated facial expression by the streamer or individual in question, and the KEKW emote is a recently popularized way to convey laughter or hilarity in the chat.