WordNet records no synonym and no antonym for megillah. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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 long boring tediously detailed account
“he insisted on giving us the whole megillah”
(Judaism) the scroll of parchment that contains the biblical story of Esther; traditionally read in synagogues to celebrate Purim
Synonyms, antonyms and the broader and narrower relations for megillah come from Princeton University’s WordNet. Full sources and licences.