[syn: knocker, doorknocker, rapper]
The Collaborative International Dictionary of English v.0.48:
knocker \knock"er\, n.
   1. 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. --Pope.
      [1913 Webster]
   2. A person strikingly handsome, beautiful, or fine; one who
      wins admiration; a "stunner." [Slang.]
      [Webster 1913 Suppl.]
   3. A species of large cockroach, especially Blabera
      gigantea, of semitropical America, which is able to
      produce a loud knocking sound.
      [Webster 1913 Suppl.]
   4. [usually used in pl.] a woman's breast. [vulgar]
      [PJC]
WordNet (r) 3.0 (2006):
knocker
    n 1: (Yiddish) a big shot who knows it and acts that way; a
         boastful immoderate person
    2: a person who knocks (as seeking to gain admittance); "open
       the door and see who the knocker is"
    3: one who disparages or belittles the worth of something [syn:
       detractor, disparager, depreciator, knocker]
    4: either of two soft fleshy milk-secreting glandular organs on
       the chest of a woman [syn: breast, bosom, knocker,
       boob, tit, titty]
    5: a device (usually metal and ornamental) attached by a hinge
       to a door [syn: knocker, doorknocker, rapper]
Moby Thesaurus II by Grady Ward, 1.0:
57 Moby Thesaurus words for "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