A question to end the year with.

A question I have absolutely no idea of the answer to. This comes from Greg Mankiw's blog.

A professor emails me:

My students have the pleasure to use your economics textbook. I have one question: where the symbol "Y" for GDP comes from? All the others, we could detect, such as NX , NCO, etc. My students are curious, and I could not give them a good answer.

My unsatisfying response:

To be honest, I don't know. It is an old convention to use Y to denote real GDP, and I am just following that. But I don't know where or why the convention began.

So anyone who knows the answer let the rest of us in on the secret.