Idaho was nearly the name of Colorado. And the meaning still repeated for it, "gem of the mountains," was made up.
In early 1860 Congress was considering a new territory for the Pike's Peak region, and many names were floated. The Albany Evening Journal of 26 March 1860 listed "Idahoe" among them, glossed as "Gem of the Mountains." The Congressional Globe that year called Idaho an Indian name with the same meaning.
The person usually blamed for that gloss is George M. Willing, who had claimed to be the region's delegate. Boise State Public Radio reports that his election was never certified and he never took office. The tale comes from Willing's own account, which his friend William O. Stoddard published in the New York Tribune in 1875.
Wordorigins calls the gloss false and says Willing likely invented it when asked what the name meant. Willing also claimed to have coined the word itself. The Museum of Hoaxes doubts that, since the claim surfaced some fifteen years later from a self-promoter. Either way, none of these sources accepts the meaning.
The name lost its first home at the last minute. When Congress created the territory on 28 February 1861, it was named Colorado instead.
But the word had already escaped. By December 1861 Washington Territory had an Idaho County, roughly where the state is now. On 3 March 1863 Congress created Idaho Territory. It became the 43rd state on 3 July 1890.
So where did the word come from? Both Etymonline and Wordorigins point to the Kiowa-Apache. Their word ídaahé meant "enemy," and they used it for the Comanche, whose lands included eastern Colorado. Not everyone agrees: Boise State Public Radio says the name is not a Native American word at all.
Later explanations piled up. Joaquin Miller gave several accounts, including "the light or diadem on the line of the mountain." Wordorigins sees a Shoshone reading, "sun coming down the mountain," as assembled after the fact. The Shoshone lived in Idaho, not in Colorado, where the name began.