Off a record by the Cattanooga Cats from their cartoon.
So if you're in your birthday suit, you're naked, right? I'm not sure how they mean it in this song. She wants to show some guy her birthday suit, but I feel like in the cartoon, she might actually have some fancy dress that's she's gonna wear on her birthday. Either way, I'm pretty sure this is all about having a good self-image and being comfortable with your body.