The Collaborative International Dictionary of English v.0.48:

Impious \Im"pi*ous\, a. [L. impius; pref. im- not + pius piou.
   See Pious.]
   Not pious; wanting piety; irreligious; irreverent; ungodly;
   profane; wanting in reverence for the Supreme Being; as, an
   impious deed; impious language.
   [1913 Webster]

         When vice prevails, and impious men bear away,
         The post of honor is a private station.  --Addison.

   Syn: Impious, Irreligious, Profane.

   Usage: Irreligious is negative, impious and profane are
          positive. An indifferent man may be irreligious; a
          profane man is irreverent in speech and conduct; an
          impious man is wickedly and boldly defiant in the
          strongest sense. Profane also has the milder sense of
          secular. --C. J. Smith. -- Im"pi*ous*ly, adv. --
          Im"pi*ous*ness, n.
          [1913 Webster]

WordNet (r) 3.0 (2006):

impious
    adj 1: lacking piety or reverence for a god [ant: pious]
    2: lacking due respect or dutifulness; "impious toward one's
       parents"; "an undutiful son" [syn: impious, undutiful]