expression, ichidan verb
drink cup after cup; have many drinks
Idiomatic expression for drinking alcohol in succession, typically sake, implying a convivial or prolonged drinking session.
宴会では、みんな杯を重ねてすっかり酔っ払った。
At the party, everyone drank cup after cup and got completely drunk.
Compound of 杯 (sake cup) + を + 重ねる (to pile up, repeat). Literally 'to pile up cups', idiomatically meaning to drink repeatedly.