I won't breath a word of it

Imagine you are in a very delicate situation: you just found out you friend is cheating on his wife. You know you don't have anything to do with it, but his wife is also a very close friend of yours and you feel like it's such a betrayal to keep that secret. When you're almost telling her about the truth, another friend comes along and makes you swear to God you are not telling anybody anything. He tells you it's not your business and that you're putting their wedding at risk. For a moment, you thought telling the truth was the right thing you should do, but then you realized you never wanna be responsible for the end of a relationship. So you say: I won't breath a word of it.

When you say that, you're assuring you won't tell anyone a secret. That expression is the same as "I won't tell a soul". You can use both in the same situation. So from now on, every time you want to let other people know you will keep a secret, you say "you won't breath a word of it", or "you won't tell a soul". I hope this helps. Take care!