背後插一刀

Simplified: 背后插一刀

bèihòuchāyīdāo ㄅㄟˋ ㄏㄡˋ ㄔㄚ ㄧ ㄉㄠ

    • to stab someone in the back
    • to attack or slander someone secretly

Character by character

Saying俗语俗語

Idiom惯用语 · In everyday use

Literally
stick a knife in someone's back
Meaning
To secretly attack or slander someone when they are off guard. Critical. Same as English 'stab in the back'.

Sources