To keep one's plans, thoughts, or intentions secret; to be discreet and not reveal information that could be used against you.
This phrase comes from card games where players literally hold their cards close to their chest to prevent opponents from seeing them. The physical action of protecting your hand from prying eyes became a metaphor for maintaining secrecy and strategic advantage in any competitive situation.
The physical gesture this phrase describes is so instinctive that even novice card players do it automatically, which explains why the metaphor feels so natural when applied to keeping secrets. Interestingly, the phrase emphasizes the 'chest' rather than just 'close to you' because the chest represents the heart - suggesting that truly important secrets should be kept closest to what matters most.
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