According to THE William Beckett, of The Academy Is..., it stands for the "Make-Up Caked, Open-Mouth-Gum-Chewing Cool Girl" that you went to school with... We all know who he is talking about. It was in refernece to the movie “Smart People” with Ellen Page in it.
direct quote from the movie:
Ellen Page: “What is it like being stupid?”
MUCOMGCCG: “It’s like sitting alone at lunch every day.”