Please tell me what happened. I promise I won't breathe a word.
(not) 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.
(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.