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!!!