The Collaborative International Dictionary of English v.0.48:

Indecorum \In`de*co"rum\, n. [Pref. in- not + decorum: cf. L.
   indecorous unbecoming.]
   [1913 Webster]
   1. Lack of decorum; impropriety of behavior; that in behavior
      or manners which violates the established rules of
      civility, custom, or etiquette; indecorousness.
      [1913 Webster]

   2. An indecorous or unbecoming action. --Young.

   Syn: Indecorum is sometimes synonymous with indecency; but
        indecency, more frequently than indecorum, is applied to
        words or actions which refer to what nature and
        propriety require to be concealed or suppressed.
        Indecency is the stronger word; indecorum refers to any
        transgression of etiquette or civility, especially in
        public.
        [1913 Webster]

WordNet (r) 3.0 (2006):

indecorum
    n 1: a lack of decorum [syn: indecorum, indecorousness]
         [ant: decorousness, decorum]
    2: an act of undue intimacy [syn: familiarity, impropriety,
       indecorum, liberty]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "indecorum":
   Babbittry, aberrance, aberrancy, abnormality, bad taste, blooper,
   boner, boo-boo, bourgeois taste, break, camp, campiness,
   criminality, delinquency, deviance, deviancy, faux pas, gaffe,
   high camp, illegality, improperness, impropriety,
   inappropriateness, incorrectness, indecency, indecorousness,
   indelicacy, indiscreetness, indiscretion, inelegance, inelegancy,
   infraction, kitsch, low camp, philistinism, poor taste, pop,
   pop culture, sinfulness, solecism, tastelessness, unaestheticism,
   unaestheticness, unbecomingness, unchastity, unfitness,
   unfittingness, unlawfulness, unmeetness, unrighteousness,
   unseemliness, unsuitability, unsuitableness, violation,
   vulgar taste, vulgarism, vulgarity, vulgarness, wickedness, wrong,
   wrongfulness, wrongness