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]