This idiom instructs someone to keep information confidential, private, or secret, as if hiding it beneath a hat where it cannot be seen. It conveys a need for discretion, often involving sensitive or privileged knowledge, and is used in personal, professional, or conspiratorial contexts to emphasize trust and secrecy. The phrase carries a tone of urgency, camaraderie, or caution, reflecting cultural values of loyalty and the protection of private matters. It resonates in settings where confidentiality is paramount, such as business deals or personal confidences, and underscores the intimacy of shared secrets and the responsibility to guard them.

The phrase likely emerged in 19th-century Britain, tied to the cultural significance of hats as personal, concealable items, often used to hide small objects in folklore and literature. An early use appears in an 1840 *Punch* magazine article, urging someone to ‘keep it under your hat’ about a scandal. The idiom gained traction in Victorian England, reflecting a society obsessed with propriety and discretion, as seen in Charles Dickens’ *Bleak House* (1853), where secrets drive the narrative. Its use grew in 20th-century American English, particularly in journalism and detective fiction, amplified by Dashiell Hammett’s *The Maltese Falcon* (1930), where secrecy is central. The phrase’s adoption was fueled by the rise of espionage and corporate confidentiality, ensuring its spread across English-speaking cultures, from Victorian parlors to modern boardrooms.