missouri has 4 distinct synonyms spread across 2 of its 4 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.
a midwestern state in central United States; a border state during the American Civil War, Missouri was admitted to the Confederacy without actually seceding from the Union
the longest river in the United States; arises in Montana and flows southeastward to become a tributary of the Mississippi at Saint Louis
“The Missouri and Mississippi Rivers together form the third longest river in the world”
a member of the Siouan people formerly inhabiting the valley of the Missouri river in Missouri
a dialect of the Chiwere language spoken by the Missouri
Synonyms, antonyms and the broader and narrower relations for missouri come from Princeton University’s WordNet. Full sources and licences.