breathe a word

To share information that is supposed to be kept secret. Often used in the negative to encourage silence. And if you breathe a word of this to the cops, we'll come after you. I was told not to breathe a word, but I knew I had to tell someone about such serious allegations. Don't breathe a word of Jenny's surprise party to her younger brother, or he'll ruin it—he's such a blabbermouth!

(not) breathe a ˈword (about/of something) (to somebody)

(not) tell somebody something, especially something secret: Please don’t breathe a word of this to anyone.