an acronym used to as a quick comeback. It stands for "Your Mom Adores Cock". Many people have never heard of it, but it is quickly making its way around in the lingo of everyday teenagers. Also, it isn't spelled out when said, "Y-M-A-C", instead it is used as a single word with the "Y" kept out, "Y-MAC".
Mike: "you fucking blow"
Hawk: "Oh yea? Well, YMAC bitch!"