(adjective)
immoral, evil, or wicked, in violation of some standard, or in the case of language, profane or obscene;
e.g. I have never seen such a foul deed in my life. He was suspended for using foul language.
(adjective)
very unpleasant, offensive, or disagreeable to the senses, soiled or contaminated;
e.g. There was a foul odor lingering in the room.
(noun)
a stroke or play in a sport that is deemed invalid or unfair, typically because it interferes with the opponent;
e.g. His move was declared a foul and he was given a warning by the referee.
(adverb)
in a way that is unfair and in violation of the rules;
e.g. if you kick the ball foul, any points scored are not counted.
(verb)
to pollute something and make it dirty or disgraced;
e.g. Many factories foul the water body by releasing chemicals into it.