a male or female who happens to be incredibly large and most of the time have a foul stench hence mcnasty. you can usually find these people at bars, ctreet corners, parties, and dances. there is a small chance that you could see a fatty mcnasty at a show. those are referred to as scene fatty mcnasty. those are clearly the worst type of fatty mcnasty's.
me: damn did you see Aly?
guy1: yeah. shes gross. i hear shes scene fucked everyone in every band in this showcase.
guy2:jeez! thats one fatty mcnasty right therrr!