backbiting (n.)

c. 1200, bacbitunge, "the sin of secretly attacking one's character or reputation through envy," from back (adj. or n.) + biting. Related: back-bite (v.) early 14c.; back-biter (c. 1200). The notion is of injury in a manner comparable to biting from behind. As an adjective Old English had bæcslitol; another old word for it was back-wounding (c. 1600).