杀回马枪

Traditional (Taiwan): 殺回馬槍

shāhuímǎqiāng ㄕㄚ ㄏㄨㄟˊ ㄇㄚˇ ㄑㄧㄤ

    • to counter-attack unexpectedly
    • to turn on one's accomplices by exposing them

Character by character

Saying俗语俗語

Idiom惯用语 · Widely known

Literally
thrust back with the spear while wheeling the horse round
Meaning
From old battles, where a fleeing warrior suddenly turned on his pursuer. Figuratively, to turn on one's accomplices by exposing them, or to counter-attack unexpectedly.

Sources