To make Motherfucking Money is to make enough money that when you tell colleagues of equal level your salary, they say "... motherfucker" in disbelief and jealousy.
Tom "Yo, tony, how much money do you make a year"
Mr. Anthony "I dont know if you'd wanna know"
Tom "come on, tell me"
Mr. Anthony "I make a 3 million dollars a year"
Tom, who makes 80k a year says ".... motherfucker"
Mr Anthony makes some motherfucking money