Meaning
To keep something confidential, telling no one else.
It’s usually said as an informal request or instruction, often about information that isn’t officially secret but shouldn’t be spread around yet.
Origin
The exact origin is undocumented, and several popular explanations turn out to be unconfirmed folklore rather than established fact: stories about medieval archers keeping spare bowstrings dry under their hats, or about Abraham Lincoln storing papers inside his stovepipe hat, are both chronologically plausible at best and lack solid supporting evidence for this specific idiom. The earliest verified use in the sense of “keep secret” is American, appearing in the trade publication “Gleanings in Bee Culture” on October 15, 1892: “we will keep it ’under our hat’ if you say so.” Slightly earlier British examples from the 1880s use similar hat imagery but in the different sense of keeping one’s own private thoughts or talents to oneself, suggesting the phrase’s meaning may have narrowed toward secrecy specifically as it crossed the Atlantic.
Examples
I’ll tell you the news, but keep it under your hat.
She kept the promotion under her hat until it was official.
Keep this under your hat for now.
A: “Did you hear about the merger?” B: “Yes, but keep it under your hat.”
Frequently asked questions
What does 'Keep it under your hat' mean?
To keep something confidential, telling no one else.
It’s usually said as an informal request or instruction, often about information that isn’t officially secret but shouldn’t be spread around yet.