backbiting

noun
  1. 1

    The action of slandering a person without that person's knowledge.

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.

adjective
  1. 1

    Slanderous or speaking badly, especially of a person without that person's knowledge.

Source: https://en.wiktionary.org/wiki/backbiting