a knife in the back

A grievous or supreme act of treachery or betrayal. (Usually preceding "of/for (someone).") This revelation regarding the governor's longstanding misappropriation of public funds is sure to be a knife in the back of his constituency, as well as every taxpayer in the state. It felt like a knife in the back to hear that Paul was going out with my ex-girlfriend. Wait, Sally—my good friend, Sally—is the one who told you not to hire me? Well, that's a knife in the back!