a stab in the back

a treacherous act or statement

傷人的暗箭；背叛的行為（或言語）

stab someone in the back

betray someone

背叛（某人）

a stab in the dark

stab (someone) in the back

To harm (someone) by treachery or betrayal of trust.

背叛（某人）或辜負其信任