knocker

Concise Oxford English Dictionary © 2008 Oxford University Press:
knocker/ˈnɒkə(r)/

▶noun

• 1 an object hinged to a door and rapped by visitors to attract attention.

  ■ informal a person who buys or sells from door to door.

• 2 informal a person who continually finds fault.

• 3 (knockers) informal a woman's breasts.

– phrases
on the knocker informal

• 1 Brit. going from door to door.

• 2 Austral./NZ (with reference to payment) without delay; immediately.