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