not breathe a word
From Longman Dictionary of Contemporary Englishnot breathe a wordnot breathe a wordSECRETto not tell anyone anything at all about something, because it is a secret Don’t breathe a word; it’s supposed to be a surprise. → breatheExamples from the Corpusnot breathe a word• He did not breathe a word.• You've got to promise not to breathe a word to anyone.