Your nuddy-pants are, well, your no-pants. When you are wearing your nuddy pants you are in the nude. The background of this word is from the Georgio Nicolsen series written by Louise Rennison.
Quite literally nude-colored pants, and you know what nide-colored pants are? They are no pants. So if you are in your nuddy-pants you are in your no paints, i.e., you are naked.