The Collaborative International Dictionary of English v.0.48:

Backbite \Back"bite`\, v. t. [2d back, n. + bite.]
   To wound by clandestine detraction; to censure meanly or
   spitefully (an absent person); to slander or speak evil of
   (one absent). --Spenser.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Backbite \Back"bite`\, v. i.
   To censure or revile the absent.
   [1913 Webster]

         They are arrant knaves, and will backbite. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

backbite
    v 1: say mean things [syn: backbite, bitch]

The Devil's Dictionary (1881-1906):

BACKBITE, v.t.  To speak of a man as you find him when he can't find
you.