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