Chubbin Muffin
Chubbin means to eat.

and 

Muffin means to cup your hand around your anus when you fart and waft it towards your friends/enemy/innocent by-standers mouth/nose
 

when you put them together it literally means to eat a fart
Gerald:*farts*
Brandon:Dude did you just fart?
Gerald:*in the background cupping his hand and moving his fart towards Brandons face*
Brandon:WTF!!What are you doing Gerald?!?!
Gerald:*ROFLcopters*Umm...you'll see.
Brandon:*Gags*WTF dude you just made me chubb a muffin!!!