A small barrel or keg, or a joker or troublemaker (British slang).
Possibly from Middle Dutch 'kagge,' a small barrel. The slang meaning of a joker or prankster may derive from the idea of someone who 'tips over the cag' or causes disruption.
In British slang, calling someone a 'cag' was a way to label rule-breakers and jokers—it's one of thousands of British terms that never crossed the Atlantic, making regional English vocabularies surprisingly divergent.
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