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]

Moby Thesaurus II by Grady Ward, 1.0:

234 Moby Thesaurus words for "impropriety":
   Babbittry, Gothicism, aberrance, aberrancy, abnormality, anomaly,
   atrocity, bad taste, badness, barbarism, barbarousness, blunder,
   bombasticness, bourgeois taste, breach, cacology, cacophony, camp,
   campiness, clumsiness, coarseness, colloquialism, corruption,
   crime, crime against humanity, criminality, crudeness,
   cumbrousness, deadly sin, delinquency, dereliction, deviance,
   deviancy, discourtesy, disentitlement, disorder, disorderliness,
   disorderly conduct, disruption, disruptiveness, dysphemism,
   empty claim, empty title, enormity, erroneousness, error, evil,
   excess, failure, false claim, falseness, falsity, fault, faux pas,
   felony, frowned-upon behavior, gaffe, gaucherie, genocide,
   gracelessness, grossness, guilty act, harshness, heaviness,
   heavy sin, high camp, hooliganism, horseplay,
   ill-balanced sentences, illegality, immodesty, immorality,
   imprecision, improperness, impurity, inaccuracy, inaccurateness,
   inadmissibility, inapplicability, inappositeness,
   inappropriateness, inaptitude, inaptness, inauspiciousness,
   incompatibility, inconcinnity, incongruity, incongruousness,
   inconvenience, incorrectness, indecency, indecorousness, indecorum,
   indelicacy, indiscreetness, indiscretion, inelegance, inelegancy,
   inequitableness, inequity, inexactitude, inexactness, inexpedience,
   inexpiable sin, infelicity, infraction, iniquitousness, iniquity,
   injury, injustice, inopportuneness, inopportunity, intempestivity,
   interruption, intrusion, invalid claim, irregularity, irrelevance,
   irrelevancy, kitsch, lack of claim, lack of finish, lack of polish,
   lapse, lasciviousness, lateness, leadenness, lewdness, localism,
   low camp, maladjustment, malefaction, malfeasance, malum,
   mesalliance, minor wrong, misalliance, misbehavior, misconduct,
   misdeed, misdemeanor, misdoing, misfeasance, misjoinder,
   misjoining, mismatch, mistake, mortal sin, naughtiness, no claim,
   nonfeasance, nonsanctioned behavior, offense, omission, outrage,
   peccadillo, peccancy, philistinism, pompousness, ponderousness,
   poor diction, poor taste, pop, pop culture, prematurity,
   roughhouse, roughness, rowdiness, rowdyism, rudeness, ruffianism,
   sesquipedalianism, sesquipedality, sin, sin of commission,
   sin of omission, sinful act, sinfulness, slang, slip,
   slipshod construction, stiltedness, suggestiveness, taboo word,
   tastelessness, tort, transgression, trespass, trip, turgidity,
   unaestheticism, unaestheticness, unbecomingness, unchastity,
   uncongeniality, uncouthness, undeservedness, undeservingness,
   undueness, unentitledness, uneuphoniousness, unfavorableness,
   unfitness, unfittingness, unfortunateness, ungracefulness,
   unjustness, unlawfulness, unmeetness, unmeritedness,
   unpropitiousness, unrefinement, unrighteousness, unripeness,
   unseasonableness, unseemliness, unsuitability, unsuitableness,
   untimeliness, unutterable sin, unwieldiness, vandalism, venial sin,
   vice, violation, vulgar taste, vulgarism, vulgarity, vulgarness,
   wickedness, wrong, wrongdoing, wrongfulness, wrongness