From Longman Dictionary of Contemporary Englishstab in the backstab in the backBETRAYwhen someone you thought was a friend tries to harm you → stab
Examples from the Corpus
stab in the back• Nationalists supported the Wilson/Callaghan governments for years, only to be stabbed in the back in the end.• MacQuillan had been stabbed in the back, there were no palm or fingerprints on the weapon.• It's so many stabs in the back ... It's the whole system which is tucking us up.• I was stabbed in the back because I don't have those problems with Steve. 