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 Cj forgot to wear pants to school, Cassietold me he was wearing his nuddy pants.
Quite literallynude-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.