originally a name for a group of native peoples among Chiwere (Siouan) tribes, from an Algonquian word recorded c. 1700, said to mean literally "people of the big canoes." Formed as a U.S. territory in 1812 (out of the whole of the Louisiana Purchase not admitted that year as the state of Louisiana); admitted as a state 1821.
In U.S. history, the Missouri Compromise (1820) in Congress admitted Missouri as a slave state, along with Maine as a free one, but set a line westward from the main southern boundary of Missouri above which no new states would be admitted with slavery. The expression I'm from Missouri, you'll have to show me is attested from at least c. 1880. Related: Missourian.