knocker vs big fish

knocker and big fish both are nouns.

Word	Noun	Adjective	Verb	Adverb
knocker	Yes	No	No	No
big fish	Yes	No	No	No

As nouns, big fish is a hypernym of knocker; that is, big fish is a word with a broader meaning than knocker:

knocker: (Yiddish) a big shot who knows it and acts that way; a boastful immoderate person
big fish: an important influential person

Other hypernyms of knocker include big cheese, big deal, big enchilada, big gun, big shot, big wheel, head honcho.

knocker (noun)	big fish (noun)
a device (usually metal and ornamental) attached by a hinge to a door	an important influential person
either of two soft fleshy milk-secreting glandular organs on the chest of a woman
one who disparages or belittles the worth of something
a person who knocks (as seeking to gain admittance)
(Yiddish) a big shot who knows it and acts that way; a boastful immoderate person