noun
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.
遅刻したので罰杯を飲まされた。
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.
Compound of 罰 (batsu, 'punishment') and 杯 (hai, 'cup/sake cup'), with rendaku voicing of 杯 to ぱい. The reading ばっぱい results from the assimilation of つ to っ before ぱ.