penalty drink; forfeit drink
Refers to alcohol one must drink as a punishment, typically in social drinking games or as a penalty for being late to a gathering.
I was made to drink a penalty drink because I was late.
ゲームに負けたら罰杯だよ。
If you lose the game, you have to take a forfeit drink.
Formed from 罰 “penalty” and 杯 “cup.” The reading reflects the sound changes from ばつはい to ばっぱい, with gemination and the resulting ぱ sound.