2) When things seem to point towards another dude being a genuine motherfucker but he actually isn't.
Origin: In the film Horrible Bosses with Colin Farrell and Jennifer Aniston, Motherfucker Jones is the murder consultant.
Dude 2: Uh-oh!
Dude 1: She was laying there, naked.
Dude 2: Dude.
Dude 1: She'd been drinking all night.
Dude 2: We get it.
Dude 1: And I snuck up behind her.
Dude 2: Aah!
Dude 1: And I slipped my fingers...into her purse.
Dude 2: Purse. You said purse! Motherfucker Jones !