The Collaborative International Dictionary of English v.0.48:

Filthy \Filth"y\, a. [Compar. Filthier; superl. Filthiest.]
   Defiled with filth, whether material or moral; nasty; dirty;
   polluted; foul; impure; obscene. "In the filthy-mantled
   pool." --Shak.
   [1913 Webster]

         He which is filthy let him be filthy still. --Rev.
                                                  xxii. 11.

   Syn: Nasty; foul; dirty; squalid; unclean; sluttish; gross;
        vulgar; licentious. See Nasty.
        [1913 Webster]

WordNet (r) 3.0 (2006):

filthy
    adj 1: disgustingly dirty; filled or smeared with offensive
           matter; "as filthy as a pigsty"; "a foul pond"; "a nasty
           pigsty of a room" [syn: filthy, foul, nasty]
    2: vile; despicable; "a dirty (or lousy) trick"; "a filthy
       traitor" [syn: dirty, filthy, lousy]
    3: characterized by obscenity; "had a filthy mouth"; "foul
       language"; "smutty jokes" [syn: cruddy, filthy, foul,
       nasty, smutty]

Moby Thesaurus II by Grady Ward, 1.0:

158 Moby Thesaurus words for "filthy":
   Fescennine, Rabelaisian, abominable, arrant, atrocious, awful,
   barfy, base, bawdy, beastly, bedraggled, beneath contempt, black,
   blameworthy, blue, brutal, coarse, contemptible, corrupt, crappy,
   cruddy, defiled, deplorable, depraved, despicable, detestable,
   dire, dirty, dirty-minded, disgusting, disheveled, dreadful,
   egregious, enormous, fecal, feculent, fetid, flagrant, flyblown,
   foul, foul-mouthed, foul-spoken, foul-tongued, fulsome, gloppy,
   grievous, grimy, gross, grubby, grungy, gunky, hateful, heinous,
   horrible, horrid, icky, immoral, impure, indecent, infamous,
   ithyphallic, lamentable, lascivious, lewd, licentious, loathsome,
   lousy, low, lurid, maggoty, malodorous, mean, mephitic, miserable,
   monstrous, mucky, nasty, nauseating, nefarious, noisome, notorious,
   obnoxious, obscene, odious, offensive, ordurous, outrageous,
   pitiable, pitiful, polluted, pornographic, puky, purulent, putrid,
   rank, raunchy, regrettable, reprehensible, repulsive, revolting,
   ribald, rotten, sad, salacious, scabby, scandalous, scatological,
   schlock, scummy, scurfy, scurrile, scurrilous, scurvy, shabby,
   shameful, shitty, shocking, shoddy, slabby, slimy, slipshod,
   sloppy, sloshy, slovenly, sludgy, slushy, smoking-room, smutty,
   soiled, soily, sordid, sposhy, squalid, stained, suggestive,
   sultry, taboo, tainted, terrible, too bad, unchaste, unclean,
   uncleanly, unkempt, unprintable, unrepeatable, unwashed, verminous,
   vile, villainous, vomity, vulgar, woeful, wormy, worst, worthless,
   wretched, yecchy