Nottingham Knockers once more

Wed 29 Jul, 16:01

One of the familiar men is knocking on doors again - I don't think he was trying to sell anything this time, the story seemed to be that he had just come out of prison.  Medium height, stocky build, dark hair, wearing shorts. 

In one way I hate to turn people away when they claim to be in need, but I let him in twice before and one time he walked in on my neighbour and started helping himself to food, and once he's in he won't shift. 

Where does the expression come from?  Just curious.