keep it under your hat

رازی را پیش خود نگه داشتن/افشا نکردن راز

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