keep something under one's hat

I didn't let on why, and told the principal to keep my call under her hat.

Just keep it under your hat that we have an active system we're going to put into this bird.

Luke, stay with the project and keep all of this under your hat.

He said you would tell me to keep it under my hat.

I hope you're going to keep all this under your hat, Chaplain.

It was a piece of information Claude seemed to be keeping under his hat, at least for the moment.

But keep it under your hat until it's certain, will you?

So I can tell you to keep it under your hat.

"Keep the whole thing under your hat if you can."

If you crack the code, keep it under your hat.