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Arapaho, an important Plains tribe of the great Algonquian family, closely associated with the Cheyenne for at least a century past. They call themselves Iñunaina, about equivalent to 'our people.' The name by which they are commonly known is of uncertain derivation, but it may possibly be, as Dunbar suggests, from the Pawnee tirapihu or larapihu, 'trader.' By the Sioux and Cheyenne they are called "Blue-sky men" or "Cloud men" the reason for which is unknown.
According to the tradition of the Arapaho they were once a sedentary, agricultural people, living far to the northeast of their more recent habitat, apparently about the Red River Valley of northern Minnesota. From this point they moved southwest across the Missouri, apparently about the same time that the Cheyenne (q. v.) moved out from Minnesota, although the date of the formation of the permanent alliance between the two tribes is uncertain.
The Atsina (q. v.), afterward associated with the Siksika, appear to have separated from the parent tribe and moved off toward the north after their emergence into the plains.
The division into Northern and Southern Arapaho is largely geographic, originating within the last century, and made permanent by the placing of the two bands on different reservations. The Northern Arapaho, in Wyoming, are considered the nucleus or mother tribe and retain the sacred tribal articles, viz, a tubular pipe, one ear of corn, and a turtle figurine, all of stone.
Since they crossed the Missouri the drift of the Arapaho, as of the Cheyenne and Sioux, has been west and south, the Northern Arapaho making lodges on the edge of the mountains about the head of the North Platte, while the Southern Arapaho continued down toward the Arkansas. About the year 1840 they made peace with the Sioux, Kiowa, and Comanche, but were always at war with the Shoshoni, Ute, and Pawnee until they were confined upon reservations, while generally maintaining a friendly attitude toward the whites. By the treaty of Medicine Lodge in 1867 the southern Arapaho, together with the Southern Cheyenne, were placed upon a reservation in Oklahoma, which was thrown open to white settlement in 1892, the Indians at the same time receiving allotments in severalty, with the rights of American citizenship. The Northern Arapaho were assigned to their present reservation on Wind River in Wyoming in 1876, after having made peace with their hereditary enemies, the Shoshoni, living upon the same reservation. The Atsina division, usually regarded as a distinct tribe, is associated with the Assiniboin on Ft Belknap reservation in Montana. They numbered, respectively, 889, 859, and 535 in 1904, a total of 2,283, as against a total of 2,038 ten years earlier.