The Collaborative International Dictionary of English v.0.48:

Backbiting \Back"bit`ing\, n.
   Secret slander; detraction.
   [1913 Webster]

         Backbiting, and bearing of false witness. --Piers
                                                  Plowman.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "backbiting":
   abuse, animadversion, backstabbing, belittlement, calumnious,
   calumny, defamation, defamatory, depreciation, detracting,
   disparagement, invective, maligning, obloquy, reflection, scandal,
   scandalous, slander, slanderous, stricture, tale, vilifying,
   vituperation