n.[C]

1. 敲击者；敲击物
2. （敲门用的）门环，门锤
3. 【美】【口】吹毛求疵的人
4. 【俚】一对乳房[P]

n.

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.