MW11 says it's from the Shoshone word "paγinappih," meaning "cloud."
Actually, it turns out this word isn't new to me; I had a vague idea it looked familiar, so I searched old email and discovered it was proposed as a fictionary word in a long-running game of email fictionary at least three times in a four-year period, but someone knew what it meant so it was never used. But still, I like it enough to post it here, and the definition here is slightly more detailed than the last one I saw, and at any rate the etymology (which I think is cool) was previously unknown to me.
Here's a question: how many other Shoshone words have become English words? Are there any common ones? I have no idea.