backstabber

verb (used with object), back·stabbed, back·stab·bing.

1. to attempt to discredit (a person) by underhanded means, as innuendo, accusation, or the like.

n.

also back-stabber, in the figurative sens, 1839, from back (n.) + agent noun from stab (v.). The verb backstab in the figurative sense is from 1925.