The Collaborative International Dictionary of English v.0.48:

Calumniatory \Ca*lum"ni*a*to*ry\, a.
   Containing calumny; slanderous. --Montagu.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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