I’m (finally) finishing up Gulliver’s Travels by Jonathan Swift. The protagonist travels to different lands where the inhabitants are significantly different from typical humans, in different ways. In the first land, they are small (Lilliputians). In another, large.
In the final voyage, the inhabitants are horse-like beings that are served by people-shaped beings called….well…yahoos. They are brutish beasts, with unpleasant demeanors, nasty habits, and no moral sense.
Sure enough, a quick look at the dictionary suggests that Swift’s book is the origin of the word “yahoo”.