keep it under your hat
Meaning: If someone tells you a secret and you keep it under your hat, you don't tell anyone.
- I'll only tell you if you promise to keep it under your hat.
- Aunt Biddy said she couldn't possibly tell me the family secrets because she'd pledged on her mother's grave to keep them under her hat.
There are at least two theories about this idiom's origin. One says it's related to the fact that archers in medieval Europe used to keep spare strings for their bows under their hats, with the implication being that other things could also be kept under one's hat without anyone knowing about it. The other says it's related to the fact that one's head is under one's hat, and that to keep something under one's hat simply means to keep it in one's head, as opposed to spreading it around by telling everyone.
I agreed to keep it under my hat and
- never forget it
- never lose it
- never tell anyone