From pop + shop, from pop (“to pawn”).
pop shop (plural pop shops)
- (slang) A pawnshop. [from 18th c.]
1789, Harris's List of Covent-Garden Ladies, London: H. Ranger, page 92:[S]he has often refused a guinea from a Flagellant, when the next hour she has been obliged to send her watch to the pop shop to get a dinner […] .