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.
When Cjforgot to wear pants to school, Cassie told me he was wearing his nuddy pants.
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.