Not be be confused with a brothel, a knocking shop is simply a house which has been elected to be the main place of sexual intercourse by those in thier teens. Usually belonging to a couple of rarely there parents.
Billys dads' house is a knocking shop.
Kevin was very shocked to see several police cars pull up outside Alec's house. "I guess he's been seen interfering with his gnomes again" Kevin surmised.
"Nah" replied John, "The pigs were tipped off that he's been running a knocking shop."