An Etymological Dictionary of Astronomy and Astrophysics

1. A question or statement so framed as to exercise one’s ingenuity in answering it or discovering its meaning; conundrum.
   

2. A puzzling question, problem, or matter (Dictionary.com).

Etymology (EN): M.E. redel, redels, from O.E. rædels “riddle; counsel; conjecture; imagination;” cf. O.Fr. riedsal “riddle,” O.Sax. radisli, M.Du. raetsel, Du. raadsel, O.H.G. radisle, Ger. Rätsel “riddle.”

Etymology (PE): Kervas “riddle, puzzle” [Dehxodâ], Kurd. karvâs “riddle,” of unknown origin.