The Collaborative International Dictionary of English v.0.48:

Fallacious \Fal*la"cious\, a. [L. fallaciosus, fr. fallacia: cf.
   F. fallacieux. See Fallacy.]
   Embodying or pertaining to a fallacy; illogical; fitted to
   deceive; misleading; delusive; as, fallacious arguments or
   reasoning. -- Fal*la"cious*ly, adv. -Fal*la"cious*ness,
   n.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

fallaciousness \fallaciousness\ n.
   1. result of a fallacy or error in reasoning.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

fallaciousness
    n 1: result of a fallacy or error in reasoning