What is a muleskinner?

The term "muleskinner" is often used of somebody who drives or works mules. One possible etymology is being good enough with a whip to injure a mule (ow) - but this seems unlikely as mules don't literally have thicker skins than horses.

It's more likely that "skin" in this sense actually means "outsmart" - because mules tend to be smarter and less tractable than most domestic animals.

There's no real consensus on the etymology, but the colorful term is still sometimes used in the American west.