keep under one's hat
Preserve the secrecy of something, as in I'll tell you about it if you promise to keep it under your hat. This usage alludes to hiding a secret in one's head, covered by a hat. [Late 1800s]
|a children's mummer's parade, as on the Fourth of July, with prizes for the best costumes.|
|a printed punctuation mark (‽), available only in some typefaces, designed to combine the question mark (?) and the exclamation point (!), indicating a mixture of query and interjection, as after a rhetorical question.|
Dictionary.com presents 366 FAQs, incorporating some of the frequently asked questions from the past with newer queries.