To keep information secret or confidential; to not reveal something you know.
This phrase emerged in the late 19th century when men's hats were essential daily wear. The space under a hat was private and secure, making it a natural metaphor for keeping secrets safe and hidden from others. The phrase gained popularity as hat-wearing became less common, preserving the metaphor.
The phrase reflects a time when hats were so integral to identity that the space beneath them felt like an extension of one's private thoughts. Ironically, as hat-wearing declined, the metaphor became more powerful - now the 'hat' represents our most private mental space, making secrets kept 'under our hat' feel especially secure and personal.
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