Stab somebody in the back - Idioms by The Free Dictionary
stab somebody in the back
stab (one) in the back
To betray someone's confidence or trust. These companies all want to pretend like they're your friend, but they'll stab you in the back the moment it makes financial sense for them. The gangster's second in command stabbed him in the back to assume control over the entire criminal organization.
stab somebody in the ˈback,
get, etc. a stab in the ˈback (informal) do or say something that harms somebody who trusts you; be treated this way: Jane promised to support me at the meeting, but then she stabbed me in the back by supporting David instead. ▶ a ˈback-stabber, ˈback-stabbing nouns: This party is full of back-stabbers. ♢ There is always a lot of back-stabbing in academic life.