backbite

Short definition from Wiktionary

Verb

1. 1
   To make spiteful slanderous or defamatory statements about someone.
2. 2
   To attack from behind or when out of earshot with spiteful or defamatory remarks.
3. 3
   To speak badly of an absent individual.

Noun

1. 1
   One who engages in backbiting; a backbiter.