The Collaborative International Dictionary of English v.0.48:

Mendacious \Men*da"cious\, a. [L. mendax, -acis, lying, cf.
   mentiri to lie.]
   1. Given to deception or falsehood; lying; as, a mendacious
      person.
      [1913 Webster]

   2. False; counterfeit; containing falsehood; as, a mendacious
      statement.
      [1913 Webster] -- Men*da"cious*ly, adv. --
      Men*da"cious*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mendacious
    adj 1: given to lying; "a mendacious child"
    2: intentionally untrue; "a mendacious statement"

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "mendacious":
   deceitful, dishonest, equivocal, equivocating, erroneous,
   fallacious, false, fibbing, forsworn, knavish, lying, paltering,
   perjured, prevaricating, roguish, shifty, spurious, truthless,
   untruthful, unveracious, wrong



The Devil's Dictionary (1881-1906):

MENDACIOUS, adj.  Addicted to rhetoric.