The Collaborative International Dictionary of English v.0.48:

Defamatory \De*fam"a*to*ry\, a.
   Containing defamation; injurious to reputation; calumnious;
   slanderous; as, defamatory words; defamatory writings.
   [1913 Webster]

WordNet (r) 3.0 (2006):

defamatory
    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:

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