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!!!