keep something under your hat
/ˌkiːp ˌsʌm.θɪŋ ˌʌn.də jə ˈhæt/idiom
to keep something secret
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Русский аналог: «никому ни слова»
Обычно в повелительном наклонении: Keep it under your hat!
“I’m pregnant, but keep it under your hat.”
“Can you keep this under your hat for now?”
“He kept the news under his hat for weeks.”