The Collaborative International Dictionary of English v.0.48:

Slanderer \Slan"der*er\, n.
   One who slanders; a defamer; a calumniator. --Jer. Taylor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

slanderer
    n 1: one who attacks the reputation of another by slander or
         libel [syn: defamer, maligner, slanderer, vilifier,
         libeler, backbiter, traducer]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "slanderer":
   Thersites, backbiter, belittler, calumniator, caustic critic,
   cynic, debunker, decrier, defamer, deflater, depreciator,
   derogator, detractor, disparager, hatchet man, knocker, libeler,
   muckraker, mudslinger, railer, social critic, traducer



Bouvier's Law Dictionary, Revised 6th Ed (1856):

SLANDERER. A calumniator, who maliciously and without reason imputes a crime
or fault to another, of which he is innocent.
     2. For this offence, when the slander is merely verbal, the remedy is
an action on the case for damages; when it is reduced to writing or
printing, it is a libel. (q.v.)