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]

The Devil's Dictionary (1881-1906):

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


Easton's 1897 Bible Dictionary:

Backbite
In Ps. 15:3, the rendering of a word which means to run about
tattling, calumniating; in Prov. 25:23, secret talebearing or
slandering; in Rom. 1:30 and 2 Cor. 12:20, evil-speaking,
maliciously defaming the absent.