Hi That's an interesting question but I'm not sure that anyone knows the answer. It's good English in common speech and, fairly recently, I'd say it's become standard In the American game of draw poker, a player may say "I'm good" meaning that they do not wish to improve their hand by taking another card from the dealer In Australian English, at a bar, you may hear someone say 'I'm good" meaning that they don't want another drink Dave
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