The name Arkansas derives from the same root as the name for the State of Kansas.
The Kansas tribe of American Indians are closely associated with the Sioux tribes. The word is a French pronunciation of a Quapaw (a related "Kaw" tribe) word meaning "land of downriver people" or "people of the south wind".
The pronunciation of Arkansas was made official by an act of the state legislature in 1881 after a dispute between the two U.S. Senators from Arkansas. One wanted to pronounce the name /ɑrˈkænzəs/ ar-KAN-zəs and the other wanted /ˈɑrkənsɔː/ AR-kən-saw.
And now you know.