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):

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