If someone is or has a big mouth, they often say things that are meant to be kept secret.

He is/has such a big mouth. 他太多嘴了。

He went and opened his big mouth and told them the whole story. 他去了，并且多嘴多舌地把整件事告诉了他们。

I'm sorry. I wasn't supposed to tell you! Me and my big mouth! 对不起，我不该告诉你们的。我真多嘴！