The Collaborative International Dictionary of English v.0.48:

Foul \Foul\ (foul), a. [Compar. Fouler (-[~e]r); superl.
   Foulest.] [OE. foul, ful, AS. f[=u]l; akin to D. vuil, G.
   faul rotten, OHG. f[=u]l, Icel. f[=u]l foul, fetid; Dan.
   fuul, Sw. ful foul, Goth. f[=u]ls fetid, Lith. puti to be
   putrid, L. putere to stink, be putrid, pus pus, Gr. py`on
   pus, to cause to rot, Skr. p[=u]y to stink. [root]82. Cf.
   Defile to foul, File to foul, Filth, Pus, Putrid.]
   1. Covered with, or containing, extraneous matter which is
      injurious, noxious, offensive, or obstructive; filthy;
      dirty; not clean; polluted; nasty; defiled; as, a foul
      cloth; foul hands; a foul chimney; foul air; a ship's
      bottom is foul when overgrown with barnacles; a gun
      becomes foul from repeated firing; a well is foul with
      polluted water.
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            My face is foul with weeping.         --Job. xvi.
                                                  16.
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   2. Scurrilous; obscene or profane; abusive; as, foul words;
      foul language.
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   3. Hateful; detestable; shameful; odious; wretched. "The foul
      with Sycorax." --Shak.
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            Who first seduced them to that foul revolt?
                                                  --Milton.
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   4. Loathsome; disgusting; as, a foul disease.
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   5. Ugly; homely; poor. [Obs.] --Chaucer.
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            Let us, like merchants, show our foulest wares.
                                                  --Shak.
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   6. Not favorable; unpropitious; not fair or advantageous; as,
      a foul wind; a foul road; cloudy or rainy; stormy; not
      fair; -- said of the weather, sky, etc.
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            So foul a sky clears not without a storm. --Shak.
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   7. Not conformed to the established rules and customs of a
      game, conflict, test, etc.; unfair; dishonest;
      dishonorable; cheating; as, foul play.
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   8. Having freedom of motion interfered with by collision or
      entanglement; entangled; -- opposed to clear; as, a rope
      or cable may get foul while paying it out.
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   Foul anchor. (Naut.) See under Anchor.

   Foul ball (Baseball), a ball that first strikes the ground
      outside of the foul ball lines, or rolls outside of
      certain limits.

   Foul ball lines (Baseball), lines from the home base,
      through the first and third bases, to the boundary of the
      field.

   Foul berth (Naut.), a berth in which a ship is in danger of
      fouling another vesel.

   Foul bill, or Foul bill of health, a certificate, duly
      authenticated, that a ship has come from a place where a
      contagious disorder prevails, or that some of the crew are
      infected.

   Foul copy, a rough draught, with erasures and corrections;
      -- opposed to fair or clean copy. "Some writers boast of
      negligence, and others would be ashamed to show their foul
      copies." --Cowper.

   Foul proof, an uncorrected proof; a proof containing an
      excessive quantity of errors.

   Foul strike (Baseball), a strike by the batsman when any
      part of his person is outside of the lines of his
      position.

   To fall foul, to fall out; to quarrel. [Obs.] "If they be
      any ways offended, they fall foul." --Burton.

   To fall foul of or To run foul of. See under Fall.

   To make foul water, to sail in such shallow water that the
      ship's keel stirs the mud at the bottom.
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WordNet (r) 3.0 (2006):

foul ball
    n 1: (baseball) a ball struck with the bat so that it does not
         stay between the lines (the foul lines) that define the
         width of the playing field [ant: fair ball]