Source : Webster's Revised Unabridged Dictionary (1913)

Backbiter \Back"bit`er\, n.
   One who backbites; a secret calumniator or detractor.

Source : WordNet®

backbiter
     n : one who attacks the reputation of another by slander or
         libel [syn: {defamer}, {maligner}, {slanderer}, {vilifier},
          {libeler}, {traducer}]