Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

fallaciousness
     n : result of a fallacy or error in reasoning