A traitor or hypocrite, such as a co-worker or friend assumed trustworthy but who figuratively attacks when one's back is turned.(noun)
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Use "backstabber" in a sentence
"Not being a backstabber is a good start, but not enough after the Straight Talk Trainwreck."
"If the backstabber is your boss, demand you create "clarification procedures" for fixing the problem that are put on record somewhere."
"If the backstabber is a colleague, not only confront the offender, but request a private meeting with your boss -- and show them the offending "knife evidence" in a calm, rational way."