In call of Duty Modern Warfare 2, the Fat Motherfucker has been named by it's inventor Valmer, for the "Juggernuts", the description was called upon the "Juggernuts" because of the amount of clothes they wear which make called by the name, Fat Motherfuckers.
Player # 1: Hey, where is the Fat Motherfucker?
Player # 2: He's hereee!, kill 'em Fat Motherfuckers
Player # 3: I just got raped by one of them, Fuck 'em
Player # 1: Wooohoo, we fucked 'em Motherfuckers ;)
when you're either in a rap battle or fighting with a homeless man for his big mac but run out words to say so you just say this. There's a 50/50 chance it will work and not.
if it does they will go home and cry in their basement in their moms house OR they get made and uppercut you balls to oblivion
homeles man:yo you want to steal my big mac from ME,well you meesed with the wrong broke man
you:(oh shit i cant think of anything,oh i know)you BIG FAT GAY MOTHERFUCKER CANT BEAT ME.
(homeless man procced to uppercut your balls and steals your car)
Fat is just rude. Rats are disgusting. So Fat Rat goes fine together, but doesn't sound right. Therefore Fat Rat motherfucker just another level of Fat Fuck...just worse
Say your boss pisses you off to the point in which the word fat or fat fuck, just doesn't settle with you. Fat Rat Motherfucker is pretty insulting, personally i don't think anyone would like to be called a rat or fat rat either, so Fat Rat Motherfucker just makes it more fun to say.