knocker
noun
[ ˈnɒkə ]
• a metal or wooden object hinged to a door and rapped by visitors to attract attention and gain entry.
• a person who continually finds fault.
• a woman's breasts.
• (in folklore) a spirit or goblin believed to inhabit mines and indicate the location of ore by knocking.
on the knocker
• going from door to door canvassing, buying, or selling.
• (of payment) immediately; on demand.
• "he has to pay cash on the knocker"
