The Collaborative International Dictionary of English v.0.48:

Indecorous \In`de*co"rous\ (?; 277), a. [L. indecorous. See
   In- not, and Decorous.]
   Not decorous; violating good manners; contrary to good
   breeding or etiquette; unbecoming; improper; out of place;
   as, indecorous conduct.
   [1913 Webster]

         It was useless and indecorous to attempt anything more
         by mere struggle.                        --Burke.

   Syn: Unbecoming; unseemly; unbefitting; rude; coarse;
        impolite; uncivil; ill-bred.
        [1913 Webster]

WordNet (r) 3.0 (2006):

indecorous
    adj 1: lacking propriety and good taste in manners and conduct;
           "indecorous behavior" [syn: indecorous, indelicate]
           [ant: decorous]
    2: not in keeping with accepted standards of what is right or
       proper in polite society; "was buried with indecent haste";
       "indecorous behavior"; "language unbecoming to a lady";
       "unseemly to use profanity"; "moved to curb their untoward
       ribaldry" [syn: indecent, indecorous, unbecoming,
       uncomely, unseemly, untoward]

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "indecorous":
   Doric, aberrant, abnormal, abominable, atrocious, barbaric,
   barbarous, cacophonous, chintzy, clumsy, coarse, criminal, crude,
   delinquent, deviant, discourteous, disgraceful, doggerel,
   dysphemistic, evil, graceless, gross, hardly the thing, harsh,
   ignominious, ill-mannered, illegal, immodest, impolite, improper,
   impure, in bad taste, inappropriate, inconcinnate, inconcinnous,
   incorrect, indecent, indelicate, indiscreet, inelegant, infamous,
   infelicitous, irregular, loose, low, malodorous, not done,
   not the thing, off-base, off-color, offensive, out-of-line,
   outlandish, ridiculous, rough, rude, sacrilegious, scandalous,
   shameful, shameless, sinful, tasteless, terrible, unbecoming,
   unbefitting, unbeseeming, uncivil, uncourtly, uncouth, undecorous,
   undignified, undue, uneuphonious, unfelicitous, unfit, unfitting,
   ungenteel, ungodly, ungraceful, unlawful, unpolished, unrefined,
   unrighteous, unseemly, unsuitable, untasteful, untoward, vulgar,
   wicked, wrong, wrongful