an act that violates the rules of a sport
"The referee called a foul on the defender for tripping the opponent during the match."
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.
"The referee blew the whistle and issued a foul after the defender tripped the runner while chasing him down the track."
In plain English: A foul is an illegal move that breaks the rules of a game and usually results in a penalty for the person who did it.
"The foul was called because he tripped over his own feet during play."
hit a foul ball
"The batter swung and missed, sending the bat flying into the stands where it was declared a foul ball."
commit a foul; break the rules
"The soccer player was shown a red card after he committed a serious foul by tripping an opponent."
To make dirty.
"The spilled milk fouled the clean carpet."
highly offensive; arousing aversion or disgust
"a disgusting smell"
"distasteful language"
"a loathsome disease"
"the idea of eating meat is repellent to me"
"revolting food"
"a wicked stench"
offensively malodorous
"a foul odor"
"the kitchen smelled really funky"
violating accepted standards or rules
"a dirty fighter"
"used foul means to gain power"
"a nasty unsporting serve"
"fined for unsportsmanlike behavior"
(of a baseball) not hit between the foul lines
"The batter swung and missed, sending the ball into the stands as it was ruled foul because it landed outside the foul line."
Covered with, or containing unclean matter; dirty.
"The muddy riverbank was foul with algae and rotting leaves."
In plain English: Foul means dirty, unpleasant, or offensive to smell and taste.
"The referee called foul for tripping the player during the game."
Usage: Use "foul" as an adjective primarily when describing something physically dirty or contaminated rather than morally bad. In sports contexts, it specifically refers to a rule violation involving illegal contact between players.
The word "foul" comes from Old English and originally meant dirty or unclean. It traveled into modern usage with this same core sense of being spoiled or offensive.