"Poppins" was a word from 'old-time' English, removed from the dictionary by the US Government in a conspiracy against the native English Language. It was once used as an adjective, and described pleasurable or nice things.
"I just got a waxymiggins off some chick last night, it was totally poppins!"
Jackson: "Did you see the filmlast night?"
Jon: "Yeah, it was totally poppins!"
Poppins Box is a term to describe female genitalia. This term makes reference to the movie Mary Poppins who carries around a bag of infinite internal proportions. She is seen removing an entire coat rack amongst other large items from her bag.
This definition is etremely useful when referring to a woman with a seemingly large, deep, or otherwise spacious vagina.
My dick's so big, this bitch better have one hell of a Poppins Box.