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]