The Collaborative International Dictionary of English v.0.48:

Libelous \Li"bel*ous\ (l[imac]"b[e^]l*[u^]s), a.
   Containing or involving a libel; defamatory; containing that
   which exposes some person to public hatred, contempt, or
   ridicule; as, a libelous pamphlet. [Written also
   libellous.] -- Li"bel*ous*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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