Source : Webster's Revised Unabridged Dictionary (1913)

Slanderer \Slan"der*er\, n.
   One who slanders; a defamer; a calumniator. --Jer. Taylor.

Source : WordNet®

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