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]