won't breathe a word

(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.
See also: breathe, word