The Collaborative International Dictionary of English v.0.48:

Impiety \Im*pi"e*ty\ ([i^]m*p[imac]"[-e]*t[y^]), n.; pl.
   Impieties ([i^]m*p[imac]"[-e]*t[i^]z). [L. impietas, fr.
   impius impious; cf. F. impi['e]t['e]. See Impious,
   Piety.]
   1. The quality of being impious; lack of piety; irreverence
      toward the Supreme Being; ungodliness; wickedness.
      [1913 Webster]

   2. An impious act; an act of wickedness.
      [1913 Webster]

            Those impieties for the which they are now visited.
                                                  --Shak.

   Syn: Ungodliness; irreligion; unrighteousness; sinfulness;
        profaneness; wickedness; godlessness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

impiety
    n 1: unrighteousness by virtue of lacking respect for a god
         [syn: impiety, impiousness] [ant: piety, piousness]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "impiety":
   blasphemousness, blasphemy, desecration, godlessness, irreligion,
   profanation, profaneness, profanity, sacrilege, sacrilegiousness,
   sinfulness, un-Christliness, unangelicalness, unchristianliness,
   ungodliness, unholiness, unrighteousness, unsaintliness,
   wickedness



The Devil's Dictionary (1881-1906):

IMPIETY, n.  Your irreverence toward my deity.