Meaning: to attack someone unfairly (definition). 


Example: As a co-worker reporting discreetly to the boss, she was a backstabber scuttling his career.
Origins: From 1920’s, one who acts behind someone’s back or “stabs your back.” (Source 1. Source 2.) 


Rewrites:  a blindside attacker an ear thumper a kidney puncher a heel scraper or crusher a ponytail dipper or grabber
Discussion: I struggled to find something closer to “back stabber” but didn’t come up with anything. Perhaps you will?