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.

A nasty ass fat chick