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):

mendaciously
    adv 1: in a mendacious and untruthful manner; "I told him, quite
           untruthfully, that I had just returned from leave" [syn:
           mendaciously, untruthfully] [ant: truthfully]