That which is so dense and secretive as to be totally indecipherable or impossible to foretell. It is a shortened version of 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. I can't make any sense of this calculus textbook, it's like a riddle wrapped in an enigma. I just don't have any idea what Mary expects from me; she's a riddle wrapped in an enigma.