The North of England had many such intriguing sayings.
I remember once entering a room in a public building in Liverpool and, not finding what I expected, turning round to go, only to be called back with "Come in, it's a shop".
This apparently referred to the custom of some tenants on council estates who would apply for licences to turn the front rooms of their homes into shops which served the immediate community.
Planning laws forbade them from altering the appearance of their homes in any way, so some of them hung a temporary sign on their door saying "Come in, it's a shop" - which became a popular catch-phrase and added to the rich store of word-lore of Liverpool.
Jim Carroll