So called because the toast in military kitchens were nicknamed shingles, as in roofing tiles. Evocative of bad cooking, which I'm betting was rampant.

Honestly, shit on a shingle (s.o.s. appropriately) is better than it sounds, even when not referred to under that name. But it's definitely a comfort food. It's not good for you, it's just creamy, beefy, and starch. Inoffensive, cheap, and easy to make in bulk. (Kinda want some now.)