1
general::
Phrase(s): (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.
McGrawhill's American Idioms And Phrasal Verbs