The Collaborative International Dictionary of English v.0.48:

Impropriety \Im`pro*pri"e*ty\, n.; pl. Improprieties. [L.
   improprietas; cf. F. impropri['e]t['e]. See Improper.]
   1. The quality of being improper; unfitness or unsuitableness
      to character, time place, or circumstances; as,
      impropriety of behavior or manners.
      [1913 Webster]

   2. That which is improper; an unsuitable or improper act, or
      an inaccurate use of language.
      [1913 Webster]

            But every language has likewise its improprieties
            and absurdities.                      --Johnson.
      [1913 Webster]

            Many gross improprieties, however authorized by
            practice, ought to be discarded.      --Swift.
      [1913 Webster]

WordNet (r) 3.0 (2006):

impropriety
    n 1: an improper demeanor [syn: impropriety, improperness]
         [ant: correctitude, properness, propriety]
    2: the condition of being improper
    3: an indecent or improper act [syn: indecency, impropriety]
    4: an act of undue intimacy [syn: familiarity, impropriety,
       indecorum, liberty]