Something that is extremely difficult, complicated, or puzzling; a problem that does not have an easy or intuitive solution. A: "We'll need to figure out how to reach that velocity without compromising the structural integrity of the vehicle." B: "It's a stumper, to be sure. I'll go back to the drawing board and see what I can come up with." I've been working on this crossword puzzle all morning long, but it's a real stumper.