Most of the words surrounding guitars seem like they come from analogies to the real world. “Head,” “neck,” and “body” are obvious. “Bridge” is a bit of a stretch, but still makes sense. But the word capo is a strange one. It has no parallel in the real world, so I went to find out where it comes from. Off to the Online Etymology Dictionary!
Turns out “capo,” like a lot of other musical terms, comes from Italian. The Italian word capo literally means “head.” They say capo tasto to refer to a capo, meaning approximately “head of the fingerboard.” So there you have it. I guess since “head” was already taken, we had to come up with something new.