under hat

under (one's) hat

In confidence; as a secret; without being disclosed to others. Keep this under your hat for now, but Greg is definitely the one getting the promotion. As long as it stays under his hat, I don't mind giving John some insider tips.
See also: hat
Farlex Dictionary of Idioms.

under (one's) hat

As a secret or in confidence: Keep this information under your hat.
See also: hat
American Heritage® Dictionary of the English Language, Fifth Edition.
See also:
  • keep something under your hat
  • 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)
  • (as) black as (one's) hat
  • titfer
  • top hat