A group of fat people that sticks up for the little guy and "takes care of" things for people if they are asked to. Fat Boy Mafia members always stick up for each other. The usual requirement to join the Fat boy Mafia is to do at least one of the following: 1) Be fat, 2) Have a high level of "phatness", 3) Do something worthy of being declared honorably fat.
Skinny guy: That guy stole my books!
Fat guy: We'll get 'em back. Don't worry, the Fat Boy Mafia's got ya back!