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]