The most extremely dirty, disgusting, or morally bad; the worst level of something unpleasant.
From Old English 'ful' meaning filthy or evil, with the superlative suffix '-est' added to form the most extreme form. The word has Germanic roots and has meant both physically dirty and morally corrupt since medieval times.
Interestingly, 'foul' was once used to describe anything considered morally wrong or socially unacceptable—including breaking the rules in sports, which is why we still say 'foul play' and 'foul ball' today.
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