keep under one's hat(redirected from under one's hat)
keep (something) under (one's) hat
To not reveal or discuss something publicly. Keep this under your hat for now, but Greg is definitely the one getting the promotion.
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]