A pornographic magazine typically with photographic images of naked ladies found new on the top shelf of newsagents managed by broad minded retailers or found second hand in waste ground. Phrase was mostly used by teenage boys in 1980s when speaking of said genre of magazine or also be used as a term to chastise their friends. The term is of Glaswegian origin and perhaps used exclusively in Scotland.
Hey you, ya nude book, what time is detention?
Hey geeza shot of that nude book that you found doon the park!
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.