We can thank the French. Arkansas was named for the French plural of a Native American tribe, while Kansas is the English spelling of a similar one. Since the letter “s” at the end of French words is usually silent, we pronounce Bill Clinton’s home state “Arkansaw.”
Kansas is named for the Kansas River, which is named for the Kansa tribe. It’s the English spelling though, so naturally, we pronounce the final “s.”
The French, however, left their mark on Arkansas‘ pronunciation. French explorers learned of a sect of the Quapaw, a Native American tribe in the territory now known as Arkansas, from the Algonquians, who called the people akansa (most likely related to the Kansa tribe).
Various French documents and books spelled the state’s name various ways — Arkancas, Akansa, Arkanceas. But “it is absolutely certain that the name as pronounced by the Indians was the same as if pronounced in our language Arkansaw,” according to the “Publication of the Arkansas Historical Association.
The “s” on the end is simply a French addition then and a silent one at that. “It is clear, then, that the name Kansas, is spelled in English, while the name Arkansas is of French orthography, and that the two names should not be pronounced alike,” according to the Arkansas Historical Association.
There’s evidence that some people tried to say the “s” (as in Ar-kan-zus) after the formation of the state government. In 1881, the Arkansas state legislature ruled on that matter, noting the “confusion” about the state’s pronunciation. Lawmakers formally endorsed the “saw” ending, discouraging any “innovation” to pronounce the state similarly to Kansas.