Some unappealing and unidentifiable meat or meat-like substance served as or incorporated into a meal. A: "Any guess what the mystery meat is they're serving in the cafeteria?" B: "If I had to guess, I'd say beef, but that is some gnarly looking beef if I'm right." In the military, you learn to choke down whatever mystery meat they give you without question. It's the only food you're going to get, so you may as well eat it.