knocker has 10 distinct synonyms spread across 3 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
(Yiddish) a big shot who knows it and acts that way; a boastful immoderate person
a person who knocks (as seeking to gain admittance)
“open the door and see who the knocker is”
one who disparages or belittles the worth of something
a device (usually metal and ornamental) attached by a hinge to a door
Synonyms, antonyms and the broader and narrower relations for knocker come from Princeton University’s WordNet. Full sources and licences.