Definitions of knocker:

• noun:   a device (usually metal and ornamental) attached by a hinge to a door
• noun:   a person who knocks (as seeking to gain admittance)

  Example: "Open the door and see who the knocker is"

• noun:   (Yiddish) a big shot who knows it and acts that way; a boastful immoderate person
• noun:   either of two soft fleshy milk-secreting glandular organs on the chest of a woman
• noun:   one who disparages or belittles the worth of something