A grievous or supreme act of treachery or betrayal. (Usually preceding "of/for (someone).")
背信弃义或背叛的严重或至高无上的行为。 (一般前述 “/对(某人)”。)
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.