Scrapple sounds like an insult, the name you call the runt in gym class. More likely it derives from the terms scraps and scrappy. And what a fitting name indeed for a traditional Pennsylvania-Dutch dish made from the odds and ends of the pig, stewed, chopped, or ground up, and mixed with cornmeal and flour. (Yum.) Often the cornmeal is cooked in the gluey gelatinous stock that comes from such a commingling of pig parts in a pot.
Once firm, scrapple is cut up and fried. It's a humble dish made with thrift in mind, just another instance of when eating low on the hog is a good thing. Think: a sort of fried polenta, were it injected with as much porky character as possible. (Although scrapple is traditionally made with pork, you could just as well use beef or lamb parts.)
What pig parts go into scrapple? That's like asking what used to go into hot dogs, and probably still does. Its contents mostly likely varied depending on what was at hand. After a pig slaughter, probably just about everything went into scrapple on the premise that as long it could be bound with mushy cornmeal and gelatinous meat stock, then it was fair game for the scrapple pot. (Meat and skin, tendons, liver, brains, little bits of stomach, ears, and so forth.)
I'm no advocate of tossing in just anything, in any amount. If you're going to use liver and/or brains, you want not to overwhelm the sweetness of the corn and the fleshy flavor of meat with too much offal.
For my first batch of scrapple, I started with two trotters, some smoked pork neck, and some liver leftover from a liver and onions dinner. I simmered the trotters and neck and ears, then ground up the cooked parts with the leftover liver. Meanwhile the cornmeal was cooking in another pot. If the cornmeal started sticking to the bottom of the pot, calling for a little more moisture, I'd dip a mug into the stockpot to retrieve a little more of that porky broth. For my second batch of scrapple, I pretty much did the same thing, except I chopped up finely some stewed pig's ears I had on hand.
Mixing up scrapple is fun. Once the grained are cooked and the pig parts processed to your desired level of coarseness, the rest is entirely up to you. (And as for cutting up the bits of pork, I happen to like it when I can still see some meaty bits in the mix, but you could process it more or less finely and still get a blockish thing of scrapple.) Like making sausage, you get to choose what spice blend you want to use. And like sausage, a slab of scrabble makes for a great accompaniment to eggs, sunny-side up and still runny.