in/wearing your birthday suit
in (one's) birthday suit
Completely naked. Alludes to the fact that one is born without clothing. I have a recurring nightmare where I'm walking through school in my birthday suit. As kids, we had no problem going swimming together in our birthday suits.
in your birthday suitINFORMAL
If you are in your birthday suit, you are not wearing any clothes. I could go out there in my birthday suit and nobody would even notice. Note: This expression is used humorously.