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]
Words nearby keep under one's hat
keep time,
keep to,
keep to oneself,
keep track,
keep under,
keep under one's hat,
keep under wraps,
keep up,
keep watch,
keep your . . .,
keeper