My heart is in my mouth

Then I said, "I hate to bug you, ma'am,
but can I have your autograph?"
& that was all that I needed
She pressed her lips against a white piece of paper
& that was all that I needed
Then I saw what she wrote, my heart is in my mouth
& that was all that I needed
- "Take, take, take", The White Stripes


What does it mean to "have your heart in your mouth"?
- This expression means that you're really scared or anxious about something. Thousands of years ago in ancient Greece, the poet Homer used the phrase in his famous poem The Iliad, and people have been saying it ever since. When your heart starts pounding so much that you can feel a thumping in your throat, it may feel like you "have your heart in your mouth." But the good news is that your heart can't move into your throat, mouth, or anywhere else. It stays put! [Source]

Personally, I can't help relating the expression to oral fixations.