The Collaborative International Dictionary of English v.0.48:

Nasty \Nas"ty\ (n[.a]s"t[y^]), a. [Compar. Nastier
   (n[.a]s"t[i^]*[~e]r); superl. Nastiest.] [For older nasky;
   cf. dial. Sw. naskug, nasket.]
   1. Offensively filthy; very dirty, foul, or defiled;
      disgusting; nauseous.
      [1913 Webster]

   2. Hence, loosely: Offensive; disagreeable; unpropitious;
      wet; drizzling; as, a nasty rain, day, sky.
      [1913 Webster]

   3. Characterized by obscenity; indecent; indelicate; gross;
      filthy.
      [1913 Webster]

   4. Vicious; offensively ill-tempered; insultingly mean;
      spiteful; as, a nasty disposition.
      [PJC]

   5. Difficult to deal with; troublesome; as, he fell of his
      bike and got a nasty bruise on his knee. [slang]
      [PJC]

   Syn: Nasty, Filthy, Foul, Dirty.

   Usage: Anything nasty is usually wet or damp as well as
          filthy or dirty, and disgusts by its stickiness or
          odor; but filthy and foul imply that a thing is filled
          or covered with offensive matter, while dirty
          describes it as defiled or sullied with dirt of any
          kind; as, filthy clothing, foul vapors, etc.
          [1913 Webster]

WordNet (r) 3.0 (2006):

nasty
    adj 1: offensive or even (of persons) malicious; "in a nasty
           mood"; "a nasty accident"; "a nasty shock"; "a nasty
           smell"; "a nasty trick to pull"; "Will he say nasty
           things at my funeral?"- Ezra Pound [syn: nasty,
           awful] [ant: nice]
    2: exasperatingly difficult to handle or circumvent; "a nasty
       problem"; "a good man to have on your side in a tight
       situation" [syn: nasty, tight]
    3: characterized by obscenity; "had a filthy mouth"; "foul
       language"; "smutty jokes" [syn: cruddy, filthy, foul,
       nasty, smutty]
    4: 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]

Moby Thesaurus II by Grady Ward, 1.0:

227 Moby Thesaurus words for "nasty":
   Fescennine, Rabelaisian, abhorrent, abominable, abusive, acute,
   annoying, arrant, atrocious, awful, awkward, bad, bad-tempered,
   baleful, barfy, base, bawdy, beastly, below contempt,
   beneath contempt, bitchy, black, blameworthy, blue, boorish,
   brackish, brutal, caddish, cantankerous, churlish, cloying, coarse,
   contemptible, crabbed, cranky, crappy, critical, crotchety, crude,
   cruel, cussed, dangerous, deplorable, despicable, detestable,
   difficult, dire, dirty, disagreeable, disgusting, dreadful,
   egregious, enormous, evil, execrable, fecal, feculent, fetid,
   filthy, flagrant, flyblown, forbidding, foul, foul-mouthed,
   foul-spoken, foul-tongued, fulsome, gloppy, grievous, gross,
   grubby, gunky, harmful, hateful, heinous, high, horrible, horrid,
   icky, ignoble, ill-bred, ill-natured, ill-tempered, improper,
   impure, inconsiderate, indecent, indecorous, indelicate, infamous,
   iniquitous, insensitive, invidious, irascible, ithyphallic,
   lamentable, lewd, licentious, loathsome, lousy, loutish, louty,
   lurid, maggoty, malefic, maleficent, malevolent, malicious, malign,
   malignant, malodorous, mawkish, mean, mephitic, miasmal, miasmic,
   monstrous, mucky, nauseant, nauseating, nauseous, nefarious,
   noisome, notorious, noxious, objectionable, obnoxious, obscene,
   odious, off color, offensive, ordurous, ornery, outrageous,
   overripe, painful, pitiable, pitiful, poisonous, pornographic,
   puky, putrid, rancid, rank, raunchy, rebarbative, regrettable,
   repellent, reprehensible, repugnant, repulsive, revolting, ribald,
   risque, rotten, rude, sad, salacious, scabby, scandalous,
   scatological, schlock, scummy, scurfy, scurrile, scurrilous,
   scurvy, serious, severe, shabby, shameful, shitty, shocking,
   shoddy, sickening, slabby, slimy, sloppy, sloshy, sludgy, slushy,
   smoking-room, smutty, soily, sordid, spiteful, spoiled, sposhy,
   squalid, stinking, suggestive, sultry, surly, tactless, terrible,
   too bad, ugly, unchaste, unclean, uncleanly, unfeminine, ungenteel,
   ungentle, ungentlemanly, unladylike, unpleasant, unprintable,
   unrefined, unrepeatable, unsavory, unseemly, unsolicitous,
   untoward, vicious, vile, villainous, vomity, vulgar, weevily,
   wicked, woeful, wormy, worst, worthless, wretched, yecchy, yucky