knocker

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A hinged fixture, such as a metal ring or bar, used for knocking on a door.
• noun Vulgar Slang A woman's breasts.
• noun A goblin or dwarf said to live under the earth and direct miners to ore by knocking.

from The Century Dictionary.

• noun One who knocks.
• noun A spirit or goblin supposed to dwell in mines, and to indicate the presence of rich veins of ore by knocking.
• noun A knob, bar, or ring of metal attached to an outer door, by knocking with which persons seeking admittance can attract the notice of the inmates. It is usually so held by a hinge that it can be lifted and allowed to fall against a metal plate or stud, giving a sharp blow. It has now generally given place to the door-bell.
• noun In milling, a device attached to a flour-bolt to jar or shake it at intervals, in order to free the cloth from the flour.
• noun One who keeps up a system of ‘knocking’ or persistent attack, with ulterior motives. See knock, v. i., 4.
• noun A sort of pendant to a wig.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who, or that which, knocks; specifically, an instrument, or kind of hammer, fastened to a door, to be used in seeking for admittance.
• noun Slang. A person strikingly handsome, beautiful, or fine; one who wins admiration; a “stunner.”
• noun A species of large cockroach, especially Blabera gigantea, of semitropical America, which is able to produce a loud knocking sound.
• noun vulgar a woman's breast.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A device, usually hinged with a striking plate, used for knocking on a door.
• noun A person who knocks (denigrates) many things.
• noun slang (usually in plural) A woman's breasts.
• noun A dwarf, goblin, or sprite imagined to dwell in mines and to indicate the presence of ore by knocking particularly in Cardigan etc. in South Wales (18th..19th century).
• noun pinball A mechanical device in a pinball table that produces a loud percussive noise.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a person who knocks (as seeking to gain admittance)
• noun (Yiddish) a big shot who knows it and acts that way; a boastful immoderate person
• noun either of two soft fleshy milk-secreting glandular organs on the chest of a woman
• noun a device (usually metal and ornamental) attached by a hinge to a door
• noun one who disparages or belittles the worth of something