Source : Webster's Revised Unabridged Dictionary (1913)

Knocker \Knock"er\, n.
   One who, or that which, knocks; specifically, an instrument,
   or kind of hammer, fastened to a door, to be used in seeking
   for admittance.

         Shut, shut the door, good John ! fatigued, I said; Tie
         up the knocker; say I'm sick, I'm dead.  --Pope.

Knocker \Knock"er\, n.
   1. A person strikingly handsome, beautiful, or fine; one who
      wins admiration; a ``stunner.'' [Slang.]

Source : WordNet®

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}]
     4: either of two soft fleshy milk-secreting glandular organs on
        the chest of a woman [syn: {breast}, {bosom}, {boob}, {tit},
         {titty}]
     5: a device (usually metal and ornamental) attached by a hinge
        to a door [syn: {doorknocker}, {rapper}]