The Collaborative International Dictionary of English v.0.48:

Slanderous \Slan"der*ous\, a.
   1. Given or disposed to slander; uttering slander.
      "Slanderous tongue." --Shak.
      [1913 Webster]

   2. Embodying or containing slander; calumnious; as,
      slanderous words, speeches, or reports.
      [1913 Webster] -- Slan"der*ous*ly, adv. --
      Slan"der*ous*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

slanderous
    adj 1: (used of statements) harmful and often untrue; tending to
           discredit or malign [syn: calumniatory, calumnious,
           defamatory, denigrative, denigrating,
           denigratory, libellous, libelous, slanderous]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "slanderous":
   abusive, back-biting, backbiting, belittling, bitchy, calumniatory,
   calumnious, catty, censorious, contemptuous, contumelious,
   decrying, defamatory, deprecatory, depreciative, depreciatory,
   derisive, derisory, derogative, derogatory, detracting, detractory,
   discrediting, disparaging, libelous, maligning, minimizing,
   muckraking, pejorative, ridiculing, scandalous, scurrile,
   scurrilous, slighting, smear, vilifying