打闷棍

Traditional (Taiwan): 打悶棍

dǎmēngùn ㄉㄚˇ ㄇㄣ ㄍㄨㄣˋ

    • to club someone from behind
    • to hit someone with sudden, unexpected criticism

Character by character

Saying俗语俗語

Idiom惯用语 · Rarely used now

Literally
hit with a blow from behind
Meaning
Originally to club someone suddenly when they are unprepared. Figuratively it means to hit someone with sudden, unexpected criticism or a blow.

Sources