knocker

Short definition from Wiktionary

Noun

1. 1
   A device, usually hinged with a striking plate, used for knocking on a door.
2. 2
   A person who knocks.
3. 3
   A critic; one who disparages.
4. 4
   A person who knocks (denigrates) something.
5. 5
   A person's breast, especially if large.
6. 6
   A dwarf, goblin, or sprite imagined to dwell in mines and to indicate the presence of ore by knocking.
7. 7
   A mechanical device in a pinball table that produces a loud percussive noise.
8. 8
   A person who is strikingly handsome or otherwise admirable; a stunner.
9. 9
   A large cockroach, especially Blaberus giganteus, of semitropical America, which is able to produce a loud knocking sound.
10. 10
    A large, boulder-shaped outcrop of bedrock in an otherwise low-lying landscape, chiefly associated with a mélange.
11. 11
    One who defaults on payment of a wager.
12. 12
    A gun.
13. 13
    An undercover policeman.

Proper noun

1. 1
   A surname.