keep something under your 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.
See also: hat, keep
Farlex Dictionary of Idioms.

keep something under your hat

INFORMAL
If you keep something under your hat, you do not tell anyone else about it. Very few people know, so keep it under your hat. Look, if I tell you something will you promise to keep it under your hat? Note: This was a slogan used to promote security in Britain during the Second World War.
See also: hat, keep, something
Collins COBUILD Idioms Dictionary, 3rd ed.

keep something under your hat

keep something a secret.
See also: hat, keep, something
Farlex Partner Idioms Dictionary

ˌkeep something under your ˈhat

(informal) keep something secret: I’m going to apply for another job, but keep it under your hat for a while, will you? OPPOSITE: let the cat out of the bag
See also: hat, keep, something
Farlex Partner Idioms Dictionary
See also:
  • keep under hat
  • keep under one's hat
  • keep (something) under (one's) hat
  • keep (something) under one's hat, to
  • pass the hat
  • pass the hat (around)
  • titfer
  • hang (one's) hat on (something)
  • top hat
  • stovepipe hat
References in periodicals archive
Is it better to keep something under your hat or close to your chest?
SAD STEVE: A MAN IN TORMENT