So I did some surfing online to find out more about po-boys. I found this excerpt about po-boy sandwiches from a 1990 N.Y. Times article:
"The po' boy sandwich from New Orleans is believed to have come from that period when beggars would go around asking for food," said Susan Costner, the author of "Great Sandwiches" (Crown, 1990). "It's from the French pourboire. The poor boys would knock on doors of French convents, and rather than let them go hungry, the nuns would scrape out the larder, assembling leftovers between split loaves of French bread."
The hero, hoagy or submarine sandwich is nothing more than a modern-day version of the original po' boy, to which cooks added a layer of very thin slices of lemon for flavor.
And here is a web page with an encyclopedic explanation of what really makes a po-boy a po-boy.