Shouting in approval and excitement, usually for a person or team you support, or it can mean anything that makes you happy.
From Old French 'chiere' (face), which evolved to mean a person's mood or expression, then what makes someone's mood happy. By the 1300s in English, 'cheer' meant both to shout approval and to encourage/gladden.
The synchronized cheering of crowds—even strangers—actually releases dopamine and creates a literal neurochemical bond between people, which is why stadiums feel electric and why humans have always gathered to cheer together at rituals!
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