The Collaborative International Dictionary of English v.0.48:
Defamer \De*fam"er\, n. One who defames; a slanderer; a detractor; a calumniator. [1913 Webster]WordNet (r) 3.0 (2006):
defamer n 1: one who attacks the reputation of another by slander or libel [syn: defamer, maligner, slanderer, vilifier, libeler, backbiter, traducer]