The Collaborative International Dictionary of English v.0.48:

Slanderous \Slan"der*ous\, a.
   1. Given or disposed to slander; uttering slander.
      "Slanderous tongue." --Shak.
      [1913 Webster]

   2. Embodying or containing slander; calumnious; as,
      slanderous words, speeches, or reports.
      [1913 Webster] -- Slan"der*ous*ly, adv. --
      Slan"der*ous*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

slanderously
    adv 1: in a false and slanderous and defamatory manner; with
           slander or calumny [syn: slanderously, calumniously]