I'd never really heard that last part before.
Something online I saw seemed about as confused!
![Rolling on the floor =))](./images/smilies/24.gif)
"What is coo-ee?" asked Jack. "No idea, but they said that in the book, they shouted, "Eustace! Eustace! Coo-ee!" And so I decided I wanted to shout coo-ee.".
Is it a real expression?