won't breathe a word
not breathe a word (of something)
To keep something a complete secret; to not share some secret or private information with anyone else. Often used in the negative to encourage silence. Please don't hurt me! I swear I won't breathe a word of this to anyone! Remember, we want this to be a surprise, so don't breathe a word to Tom. He promised me that he wouldn't breathe a word of what he saw here today.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
(I) won't breathe a word (of it).and (I) won't tell a Soul.
Fig. I will not tell anyone your secret. Bill: Don't tell anybody, but Sally is getting married. Mary: I won't breathe a word of it. Alice: The Jacksons are going to have to sell their house. Don't spread it around. Mary: I won't tell a soul.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.