knocker

Etymology

knock + -er

noun

1. A device, usually hinged with a striking plate, used for knocking on a door.
   His tongue became a staircase, his uvula the knocker/Of an ornate wooden door that led me straight into my future 18 August 2016, Ben Gallaty (lyrics and music), “Small Red Boy”, in The Bible 2, performed by AJJ
2. A person who knocks.
   He was a loud knocker. Despite my usual timidity, after a bit I opened the door. 1963, Patrick Anderson, The Character Ball: Chapters of Autobiography, page 220
3. A critic; one who disparages.
   "I try to give everybody credit," declared the old man. "I am not a knocker. I am not too good for this world." 1947, Saul Bellow, The Victim
4. (informal, derogatory) A person who knocks (denigrates) something.
5. (slang, usually in the plural) A woman's breast.
6. (especially Cardigan, in South Wales, archaic) A dwarf, goblin, or sprite imagined to dwell in mines and to indicate the presence of ore by knocking.
7. (pinball) A mechanical device in a pinball table that produces a loud percussive noise.
   A good game needs color, lights, bells, gongs, and knockers, all to assure the player he is making progress[…] 1963, Harper's magazine, volume 226
8. (dated, slang) A person who is strikingly handsome or otherwise admirable; a stunner.
9. A large cockroach, especially Blaberus giganteus, of semitropical America, which is able to produce a loud knocking sound.
10. (geology) A large, boulder-shaped outcrop of bedrock in an otherwise low-lying landscape, chiefly associated with a mélange.
11. (slang) One who defaults on payment of a wager.
    To the consternation of those who believed that bookies were 'knockers' (defaulters), he paid his losses with alacrity […] 2004, Carl Chinn, Better Betting with a Decent Feller, page 48