adj

• (of words or a way of speaking) Obscene, vulgar or abusive.
  "The rascal spewed forth a series of foul words."
• (obsolete) Ugly; homely; poor.
  "Let us, like merchants, show our foulest wares."
• (nautical) Entangled and therefore restricting free movement, not clear.
  "We've got a foul anchor."
• (technical) (with "of") Positioned on, in, or near enough to (a specified area) so as to obstruct it.
  "The bus had stopped just foul of the north track at the Erindale Station Road public railway crossing […] With the bus stationary, but still foul of the north track, the train struck one of its front mirrors."

verb

• (transitive) To make dirty.
  "to foul the face or hands with mire"
• (transitive) To besmirch.
  "He's fouled his reputation."
• (transitive, intransitive) To obstruct, block, or otherwise interfere with (something), for example by clogging (a drain, gun barrel, chimney, etc) or by being in the way of (a gunshot, etc).
  "The hair has fouled the drain."
• (transitive, nautical) To entangle.
  "The kelp has fouled the prop."

noun

• (sports) A breach of the rules of a game, especially one involving inappropriate contact with an opposing player in order to gain an advantage; for example, tripping someone up in soccer, or contact of any kind in basketball.
  "A second Norwich goal in four minutes arrived after some dire Newcastle defending. Gosling gave the ball away with a sloppy back-pass, allowing Crofts to curl in a cross that the unmarked Morison powered in with a firm, 12-yard header. Gosling's plight worsened when he was soon shown a red card for a foul on Martin."
• (bowling) A (usually accidental) contact between a bowler and the lane before the bowler has released the ball.
• (baseball) A foul ball, a ball which has been hit outside of the base lines.
  "Jones hit a foul up over the screen."