The Collaborative International Dictionary of English v.0.48:

Calumnious \Ca*lum"ni*ous\, a. [L. calumniosus.]
   Containing or implying calumny; false, malicious, and
   injurious to reputation; slanderous; as, calumnious reports.
   [1913 Webster]

         Virtue itself 'scapes not calumnious strokes. --Shak.

   Syn: . Slanderous; defamatory; scurrilous; opprobrious;
        derogatory; libelous; abusive. -- Ca*lum"ni*ous*ly,
        adv. -- Ca*lum"ni*ous*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

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