The word foul has several meanings:

1. Something disgusting or offensive in smell, taste, or appearance. A foul odor might come from spoiled milk or a garbage can left too long in the sun. Foul weather means stormy, nasty conditions with dark clouds and heavy rain. When something tastes foul, it's so unpleasant you want to spit it out immediately.
2. To break the rules in sports. In basketball, you commit a foul when you push, hold, or hit another player instead of just playing the ball. In baseball, a foul ball is one hit outside the playing field's boundary lines. These fouls stop the game temporarily while referees sort things out. Players who commit too many fouls can be removed from the game.
3. To make something dirty or tangled. A boat's anchor line might foul on underwater debris, getting hopelessly twisted. A bird's nest fouls a chimney when it blocks the opening.

The word often suggests something has gone wrong or become corrupted. Foul play means someone acted dishonestly or violently. When a plan goes wrong, people sometimes say they've had a foul-up. The opposite of foul is usually fair, whether you're talking about weather, sports, or how people treat each other.