Once a librarian, always a librarian. So, my family came to visit, and it was fun to show my niece and nephews around Boston. However, just as we were finishing lunch outside Quincy Market, a man with a German accent walked up to my dad and I and asked:
German: You are Americans, yes?
German: Good, I was wondering, can you tell me, why are Americans called Yankees?
I had no idea. It is one of those questions that feels like it should have an answer that everyone knows, but I couldn't remember ever hearing where this nickname came from - and on the spot, I couldn't even come up with anything that even sounded reasonable.
So we apologized for not being able to help him. He said it was no problem and merrily walked off - but I felt kind of bad, and of course was curious.
Since I was on vacation and not at the library, all I had was the internet - and of course my first stop was Wikipedia's article for Yankee. The Origins and history of the word section starts off with "The origin of the term is uncertain," and cites some pre-Revolution instances, as well as details Faulty theories. Ultimately though, it says that a Dutch origin is the most likely scenario.
When I got back to work the following week, I checked some of our word origin books, but none that were checked in included Yankee. So, my next stop was our Oxford English Dictionary.
Although the OED also indicated [Source unascertained], it dedicated almost a full two columns to the word. And what it did say about the etymology basically echoed what I'd already found:
So, I'm sorry German tourist, but it looks like there actually isn't a good answer to your question (so, technically, my Dad and I saying we didn't know was the correct answer).
Regardless, I still felt like I should have given him a business card or something and told him I'd let him know after I I got back to work and looked it up. I guess librarians never go on vacation - just wider roving reference.