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]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "impious":
   apostate, arch, atheistic, backsliding, blasphemous, casual,
   coltish, contrary, devil-may-care, disobedient, elvish, fallen,
   fallen from grace, flippant, free and easy, fresh, frolicsome,
   froward, giddy, godless, iconoclastic, impish, iniquitous,
   irreligious, irreverent, lapsed, mischievous, offhand, pert,
   perverse, pixieish, profanatory, profane, puckish, recidivist,
   recidivistic, recreant, renegade, roguish, sacrilegious, saucy,
   scandalous, sinful, sportive, un-Christly, unangelic, unchristian,
   undevout, unduteous, undutiful, unfaithful, ungodly, unhallowed,
   unholy, unrighteous, unsaintly, waggish, wayward, wicked,
   wrongheaded