what's a hoagie?

It’s the Philadelphian name for a big Italian sandwich.

In the late '20s a jazz musician named Al De Palma saw some fellow hepcats eating a sub, and commented to himself "You’d have to be a hog" to eat a sandwich that big. So when he opened a sub shop during the depression, he started calling his big sandwiches "hoggies," he eventually opened chains across the city.  As for why "hoggie" turned to "hoagie," the best explanation is probably the Philadelphia accent itself.  Ever heard those guys talk?