One who, or that which, knocks; specifically, an instrument, or kind of hammer, fastened to a door, to be used in seeking for admittance. [1913 Webster]
"Shut, shut the door, good John ! fatigued, I said;
Tie up the knocker; say I'm sick, I'm dead." [1913 Webster]
A person strikingly handsome, beautiful, or fine; one who wins admiration; a “stunner.” [Webster 1913 Suppl.]
A species of large cockroach, especially Blabera gigantea, of semitropical America, which is able to produce a loud knocking sound. [Webster 1913 Suppl.]
a woman's breast. [PJC]

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OXFORD DICTIONARY

knocker, n.
1 a metal or wooden instrument hinged to a door for knocking to call attention.
2 a person or thing that knocks.
3 (in pl.) coarse sl. a woman's breasts.
4 a person who buys or sells door to door.

Idiom

knocker-up Brit. hist. a person employed to rouse early workers by knocking at their doors or windows. on the knocker
1 a (buying or selling) from door to door. b (obtained) on credit.
2 Austral. & NZ colloq. promptly. up to the knocker Brit. sl. in good condition; to perfection.

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THESAURUS

knocker

Thersites, backbiter, bag, belittler, boobs, bosom, breast, breasts, brisket, bust, calumniator, carper, caustic critic, caviler, chest, criticizer, crop, cynic, debunker, decrier, defamer, deflater, depreciator, derogator, detractor, disparager, dug, faultfinder, hatchet man, knockers, libeler, mama, mamelon, mamelonation, mammary gland, mammilla, mammillation, momus, muckraker, mudslinger, nenes, nipple, pap, papilla, pigeon breast, railer, slanderer, smellfungus, social critic, teat, thorax, tit, tits, titties, titty, traducer, udder

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ROGET THESAURUS

knocker

Instrument

N instrument, machinery, mechanism, engineering, instrument, organ, tool, implement, utensil, machine, engine, lathe, gin, mill, air engine, caloric engine, heat engine, gear, tackle, tackling, rig, rigging, apparatus, appliances, plant, materiel, harness, trappings, fittings, accouterments, barde, equipment, equipmentage, appointments, furniture, upholstery, chattels, paraphernalia, mechanical powers, lever, leverage, mechanical advantage, crow, crowbar, handspike, gavelock, jemmy, jimmy, arm, limb, wing, oar, paddle, pulley, wheel and axle, wheelwork, clockwork, wheels within wheels, pinion, crank, winch, cam, pedal, capstan, wheel, inclined plane, wedge, screw, spring, mainspring, can hook, glut, heald, heddle, jenny, parbuckle, sprag, water wheel, handle, hilt, haft, shaft, heft, shank, blade, trigger, tiller, helm, treadle, key, turnscrew, screwdriver, knocker, hammer, edge tool, borer, vice, teeth, nail, rope, peg, support, spoon, arms, oar, cardiograph, recapper, snowplow, tenpenny, votograph, instrumental, mechanical, machinal, brachial.

Convexity

N convexity, prominence, projection, swelling, gibbosity, bilge, bulge, protuberance, protrusion, camber, cahot, thank-ye-ma'am, swell, intumescence, tumour, tumor, tubercle, tuberosity, excrescence, hump, hunch, bunch, boss, embossment, hub, hubble, tooth, knob, elbow, process, apophysis, condyle, bulb, node, nodule, nodosity, tongue, dorsum, bump, clump, sugar loaf, bow, mamelon, molar, belly, corporation, pot belly, gut, withers, back, shoulder, lip, flange, pimple, zit, wen, wheel, papula, pustule, pock, proud flesh, growth, sarcoma, caruncle, corn, wart, pappiloma, furuncle, polypus, fungus, fungosity, exostosis, bleb, blister, blain, boil, airbubble, blob, papule, verruca, papilla, nipple, teat, tit, titty, boob, knocker, pap, breast, dug, mammilla, proboscis, nose, neb, beak, snout, nozzle, schnoz, peg, button, stud, ridge, rib, jutty, trunnion, snag, cupola, dome, arch, balcony, eaves, pilaster, relief, relievo, cameo, bassorilievo, mezzorilevo, altorivievo, low relief, bas relief, high relief, hill, cape, promontory, mull, forehead, foreland, point of land, mole, jetty, hummock, ledge, spur, naze, ness, convex, prominent, protuberant, projecting, bossed, embossed, bossy, nodular, bunchy, clavate, clavated, claviform, hummocky, moutonne, mammiliform, papulous, papilose, hemispheric, bulbous, bowed, arched, bold, bellied, tuberous, tuberculous, tumous, cornute, odontoid, lentiform, lenticular, gibbous, club shaped, hubby, hubbly, knobby, papillose, saddle-shaped, selliform, subclavate, torose, ventricose, verrucose, salient, in relief, raised, repousse, bloated, (expanded).

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