stab in the back, a
A betrayal of trust, an act of treachery, as in Voting against our bill at the last minute was a real stab in the back. It is also put as stab someone in the back, meaning "betray someone." For example, Don't trust George; he's been known to stab his friends in the back. Both the noun and verb forms of this idiom, alluding to a physical attack when one's back is turned, date from the early 1900s.