That which is so dense and secretive as to be totally indecipherable or impossible to foretell. It is from a line used by Winston Churchill to describe the intentions and interests of Russia in 1939: "I cannot forecast to you the action of Russia. It is a riddle, wrapped in a mystery, inside an enigma; but perhaps there is a key. That key is Russian national interest." Many versions, variations, and appropriations of the quote, its structure, and its meaning have since been in use. Political campaigns make my head hurt. They're just a riddle, wrapped in a mystery, inside an enigma.

Something that is difficult to understand. Why she walked out in such a huff this morning is a mystery to me! She was acting normally before that. It was a mystery to scientists where the birds migrated to until they banded some of them and tracked their movement.

To be difficult to understand or decipher. This phrase is often humorously applied to unimportant things. The living cannot truly understand death—it's another of life's great mysteries. Why dogs and cats hate each other is another of life's great mysteries.

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.