The Collaborative International Dictionary of English v.0.48:

Indecent \In*de"cent\, a. [L. indecens unseemly, unbecoming: cf.
   F. ind['e]cent. See In- not, and Decent.]
   Not decent; unfit to be seen or heard; offensive to modesty
   and delicacy; as, indecent language. --Cowper.

   Syn: Unbecoming; indecorous; indelicate; unseemly; immodest;
        gross; shameful; impure; improper; obscene; filthy.
        [1913 Webster]

WordNet (r) 3.0 (2006):

indecent
    adj 1: 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]
    2: offensive to good taste especially in sexual matters; "an
       earthy but not indecent story"; "an indecent gesture" [ant:
       decent]
    3: offending against sexual mores in conduct or appearance; "a
       bathing suit considered indecent by local standards"

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "indecent":
   besmirched, blue, chintzy, coarse, debauched, defiled, degenerate,
   dirty, distasteful, filthy, foul, gross, ill-mannered, immodest,
   improper, impure, in bad taste, inappropriate, indecorous,
   indelicate, indiscreet, inelegant, ithyphallic, lascivious, lewd,
   licentious, lubricious, lubricous, maculate, malodorous, nasty,
   obscene, offensive, outrageous, pornographic, prurient, rank,
   raunchy, repellent, repulsive, ridiculous, risque, rude, salacious,
   scatological, shameful, shameless, smirched, smutty, soiled,
   suggestive, sullied, tainted, tasteless, unbecoming, unbeseeming,
   unchaste, unclean, undecorous, undignified, unfit, unfitting,
   ungenteel, ungodly, unseemly, unsuitable, untasteful, untoward,
   unvirginal, unvirtuous, vile, vulgar