I. door ‧ knock ‧ er /ˈdɔːˌnɒkə $ ˈdɔːrˌnɑːkər/ BrE AmE ( also knocker ) noun [countable]
a heavy metal ring or bar on a door that you use to knock with
II. knock ‧ er /ˈnɒkə $ ˈnɑːkər/ BrE AmE noun [countable]
1 . a piece of metal on an outside door of a house that you hit against the door to attract the attention of the people inside ⇨ ↑ doorknocker
2 . knockers [plural] spoken not polite an offensive word meaning a woman’s breasts