According to Wikipedia, Idaho is the only state to have been named by result of a hoax. According to legend, George Willing suggested the name in the 1860s for the territory that later became Colorado. He claimed that it came from a Shoshone language and meant either 'the sun comes from the mountains', or 'gem of the mountains'. While it didn't end up becoming the name for the territory he intended it to, it did eventually come into play.
Idaho County was created as a part of the eastern part of Washington Territory in the same year. It is said that this name came from a steamship whose name also was Idaho, but it is unclear as to whether the steamship was named before or after the Willing suggested the name. Ee-dah-how is the Shoshoni word that Willing said the name Idaho came from, though later he admitted that it was not a Native American word, but was rather something he had made up of his own accord.