backbite

verb

1. To make spiteful slanderous or defamatory statements about someone.

   “They beguiled the time by backbiting and intriguing against each other in a foolish kind of way.”

   “Gumshoe: You're not gonna get me to backbite a woman with a whip. No way.”

2. (informal)To attack from behind or when out of earshot with spiteful or defamatory remarks.
3. To speak badly of an absent individual.

noun

1. One who engages in backbiting; a backbiter.