1861 as a place name, originally applied by U.S. Congress to a proposed territorial division centered in what is now eastern Colorado; said at the time to mean "Gem of the Mountains" but probably rather from Kiowa-Apache (Athabaskan) idaahe "enemy," a name applied by them to the Comanches. Modern Idaho was organized 1861 as a county in Washington Territory; in 1863 became a territory in its own right and it was admitted as a state in 1890.
Holy crap - Idaho is probably from a word meaning enemy.