Bill: Sara, do you think anyone knows about what happened? It was so embarrassing.
Sara: Don't tell Johnny anything if you want to keep this secret. He HAS a big mouth! Everyone will know what happened in a few hours!
I learned that it's not good to BE a big mouth. I learned my lesson after I got my friend in trouble. I used to gossip and tell everyone everything, whether it was true or not. One day, I heard some negative gossip about my friend. I didn't think it was true, but I told my classmates about it because I loved to gossip. Later, our classmates wouldn't talk to her because they thought the gossip was true I felt terrible because it was my fault. I apologized to my friend and I promised myself that I would never gossip again!
You can BE a big mouth OR you could HAVE a big mouth. The meaning doesn't change.
This expression describes someone who talks too much, often revealing secrets.