His heart is in his mouth what does this sentence mean?

The phrase "His heart is in his mouth" is an idiom, meaning someone is feeling extremely nervous or scared.

It's a way of describing the feeling of your heart beating so rapidly and forcefully that it feels like it's going to jump out of your chest.

Here are some other ways to say the same thing:

* He's terrified.

* He's panicking.

* He's scared out of his wits.

* He's feeling very anxious.

Let me know if you have any other idioms you'd like me to explain!