Why is the ninth month called September?
group of North American Indian tribes, 1761, from North American French, short for Nadouessioux, sometimes said to be from Ojibway (Algonquian) Natowessiwak (plural), literally "little snakes," from nadowe "Iroquois" (literally "big snakes"). Another explanation traces it to early Ottawa (Algonquian) singular /na:towe:ssi/ (plural /na:towe:ssiwak/) "Sioux," apparently from a verb meaning "to speak a foreign language" [Bright]. In either case, a name given by their neighbors; the people's name for themselves is Dakota.
A common name for the Dakota people, a tribe of Native Americans inhabiting the northern Great Plains in the nineteenth century. They were famed as warriors and frequently took up arms in the late nineteenth century to oppose the settlement of their hunting grounds and sacred places. In 1876, Sioux warriors, led by Chief Sitting Bull, and commanded in the field by Chief Crazy Horse, overwhelmed the United States cavalry at the Battle of the Little Bighorn. (See Custer's last stand.) A group of Sioux under Chief Big Foot were massacred by United States troops at Wounded Knee in 1890.