It is a Stoner originated phrase that is used to request that your friend, homie, partner, etc. pack a bowl of weed into a bong or piece (pipe), that is to small to be a snap and too big to be a full bowl.
Brother 1: Hey I'm not that faded, do you wanna smoke another bowel?
Brother 2: Well I'm a little high, but we need to save weed so don't pack a bowel. And a snap
won't really do anything, so... Pack a Tweener :)