Hat Secret Don’t In The Loop 409 One’s Richard It’s Impossible

1. Don’t tell Richard anything you don’t want everyone else to know. It’s impossible for him to keep anything under his hat.
2. I’m not telling anyone yet, but Tom and I are getting married. Keep it under your hat, okay?

Antonyms: spill the beans; let the cat out of the bag

This phrase originates from the 1800s, when many men and women wore hats. The idea is to keep a secret in your head, underneath a hat.