What does Boxing Day have to do with boxing?
any type of processed meat whose source is indistinguishable, esp. that served in a cafeteria
Spam is the most famous mystery meat.
Meat not readily identifiable, esp as served in a student dining hall, fraternity house, etc: Parents must follow the rules: They are forbidden to smoke or leave the building, must show up at homeroom and eat mystery meat at lunch like the rest of their classmates
[1970s+ Students; meat loaf was called mystery loaf by the 1940s]