Source : Webster's Revised Unabridged Dictionary (1913)

Defamer \De*fam"er\, n.
   One who defames; a slanderer; a detractor; a calumniator.

Source : WordNet®

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