The Collaborative International Dictionary of English v.0.48:

Obscene \Ob*scene"\, a. [L. obscenus, obscaenus, obscoenus, ill
   looking, filthy, obscene: cf. F. obsc['e]ne.]
   [1913 Webster]
   1. Offensive to chastity or modesty; expressing or presenting
      to the mind or view something which delicacy, purity, and
      decency forbid to be exposed; impure; as, obscene
      language; obscene pictures.
      [1913 Webster]

            Words that were once chaste, by frequent use grew
            obscene and uncleanly.                --I. Watts.
      [1913 Webster]

   2. Foul; fifthy; disgusting.
      [1913 Webster]

            A girdle foul with grease binds his obscene attire.
                                                  --Dryden
                                                  (Aeneid, vi.
                                                  417).
      [1913 Webster]

   3. Inauspicious; ill-omened. [R.] [A Latinism]
      [1913 Webster]

            At the cheerful light,
            The groaning ghosts and birds obscene take flight.
                                                  --Dryden.
      [1913 Webster]

   Syn: Impure; immodest; indecent; unchaste; lewd.
        [1913 Webster] -- Ob*scene"ly, adv. --
        Ob*scene"ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

obscene
    adj 1: designed to incite to indecency or lust; "the dance often
           becomes flagrantly obscene"-Margaret Mead
    2: offensive to the mind; "an abhorrent deed"; "the obscene
       massacre at Wounded Knee"; "morally repugnant customs";
       "repulsive behavior"; "the most repulsive character in recent
       novels" [syn: abhorrent, detestable, obscene,
       repugnant, repulsive]
    3: suggestive of or tending to moral looseness; "lewd
       whisperings of a dirty old man"; "an indecent gesture";
       "obscene telephone calls"; "salacious limericks" [syn:
       lewd, obscene, raunchy, salacious]

Moby Thesaurus II by Grady Ward, 1.0:

172 Moby Thesaurus words for "obscene":
   Cyprian, Fescennine, Paphian, Rabelaisian, abhorrent, abominable,
   abusive, amoral, animal, aphrodisiomaniacal, atrocious, awful,
   barnyard, base, bawdy, beastly, below contempt, beneath contempt,
   blasphemous, blue, broad, calumniatory, calumnious, carnal,
   clitoromaniacal, coarse, comminatory, concupiscent, contemptible,
   contumelious, crass, crude, crusty, cursing, damnatory, debauched,
   degenerate, denunciatory, despicable, detestable, dirty,
   disgusting, dissolute, distasteful, dysphemistic, earthy,
   epithetic, erotic, eroticomaniacal, erotomaniacal, evil,
   excommunicative, excommunicatory, execrable, execratory, fetid,
   filthy, fleshly, forbidding, foul, foul-mouthed, foul-spoken,
   foul-tongued, foulmouthed, fulminatory, fulsome, gaudy, goatish,
   gross, gynecomaniacal, hateful, heinous, hideous, horny, horrible,
   hot, hysteromaniacal, ignoble, immodest, immoral, imprecatory,
   improper, impure, indecent, indecorous, indelicate, inelegant,
   insufferable, intolerable, ithyphallic, lascivious, lecherous,
   lewd, libertine, libidinous, licentious, lickerish, loathsome,
   loose, loud, lubricious, lurid, lustful, maledictory, malodorous,
   mephitic, meretricious, miasmal, miasmic, nasty, nauseating,
   nauseous, noisome, noxious, nymphomaniacal, objectionable,
   obnoxious, odious, off color, offensive, outrageous, paw,
   pornographic, priapic, profane, prurient, randy, rank, raunchy,
   raw, rebarbative, repellent, repugnant, repulsive, revolting,
   ribald, rich, risque, rocky, rough, rude, ruttish, salacious,
   satyric, scabrous, scatologic, scatological, scurrile, scurrilous,
   sensual, sexual, sexy, shameful, shameless, shocking, sickening,
   smoking-room, smutty, stinking, suggestive, sultry, unchaste,
   unclean, unpalatable, unprintable, unrepeatable, vile,
   vituperative, vulgar, wanton, warm, wicked