Violations of rules in sports, or disgusting and offensive things (adjective: foul means dirty, nasty, or wicked).
From Old English 'ful,' related to 'filth.' Originally meant physically dirty, but extended to mean morally corrupt or rule-breaking violations.
A basketball player who commits too many fouls gets fouled out, which is where that expression comes from—you're literally dirty or guilty of breaking the rules too many times!
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