Source : Webster's Revised Unabridged Dictionary (1913)

Nasty \Nas"ty\, a. [Compar. {Nastier}; superl. {Nastiest}.] [For
   older nasky; cf. dial. Sw. naskug, nasket.]
   1. Offensively filthy; very dirty, foul, or defiled;
      disgusting; nauseous.

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

   3. Characterized by obcenity; indecent; indelicate; gross;
      filthy.

   Syn: {Nasty}, {Filthy}, {Foul}, {Dirty}.

   Usage: Anything nasty is usually wet or damp as well as
          filthy or dirty, and disgusts by its stickness 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.

Source : WordNet®

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: {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: {tight}]
     3: thoroughly unpleasant; "filthy (or foul or nasty or vile)
        weather we're having" [syn: {filthy}, {foul}, {vile}]
     4: characterized by obscenity; "had a filthy mouth"; "foul
        language"; "smutty jokes" [syn: {filthy}, {foul}, {smutty}]
     5: 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}]
     [also: {nastiest}, {nastier}]