當頭棒

Simplified: 当头棒

dāngtóubàng ㄉㄤ ㄊㄡˊ ㄅㄤˋ

    • a sharp warning that jolts someone awake
    • wake-up call

Character by character

Saying俗语俗語

Idiom惯用语 · Widely known

Literally
a blow on the head
Meaning
A sharp warning that jolts someone awake, from the Zen practice of striking students. Close to "a wake-up call".

Sources