A drinking game played by removing the bladder from cheap boxed wine, and holding it by the non-tap end of the bag so another player can slap the bag as hard as possible. The player then kneels or sits so the wine can be poured into their mouth by the person holding the bag.
The signal that the drinking player has had enough wine is usually to hold forth an imaginary orb of mystic power with one hand, but can be any pre-defined signal.
Even those who do not enjoy wine are usually entieced to play slappy bag as everyone wants to slap the bag.
"Slappy bag! SLAAAAPPY BAAAAAG!!!! No one can resist it!"