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.