back-stabbing 
noun [mass noun]
The action of criticizing someone in a treacherous manner despite pretending friendship with them: the media world of back-stabbing, scheming, and downright malice...
- True, we are expected to moonwalk across the vast waters dividing technology from the masses and tiptoe back on egocentric eggshells, circumventing treacherous misunderstandings and political back-stabbing.
- Yet amid the betrayal and back-stabbing, the cruelty and crime, the play ultimately has a happy, almost spiritual, finale, shaking the audience out of its 21st century complacency to ask: has anything really changed?
- After the lying and back-stabbing that took place this week, relations between the housemates have degenerated into drunken abuse and name-calling.
adjective
(Of a person) engaging in back-stabbing; critical of other people despite pretending friendship with them: back-stabbing officials protecting their patch with petty politics...
- It won't help you achieve a spurious state of spiritual well-being, or help flush toxins out of your system - but it will help you to understand what your boss, your back-stabbing colleague or that girl at the water cooler really think of you.
- He asked me how I put up with being with all those back-stabbing girls in the company who just wanted the spotlight, and I asked him how he put up with all the public school idiots and the violence.
- There was still a belief amongst many people that the army had not lost the war but had been let down by back-stabbing politicians.
Derivatives
back-stabber
- But now, you're a betrayer, and a back-stabber at that.
- When they sang about back-stabbers, they weren't just complaining about haters - they were indicting them.
- But rather than the racial and gender melting pot that he expected, he encountered a world of gangsters, thieves and back-stabbers.