Did you ever wonder where the state of Missouri got its name? If you said from the Missouri River that runs through it, you would be correct but where did the river get its name?
If you said from an Indian tribe, again you would be correct.
Both the river and the state were named after an Indian tribe, but this tribe never called themselves the Missouri.
The United States acquired the Louisiana Purchase from France in 1803. The river in Upper Louisiana had already been named the Missouri River by then and the area around it was called the Missouri Territory.