Middle English redels, ridel, from Old English rǣdelse opinion, conjecture, riddle; akin to Old English rǣdan to interpret
noun
a mystifying, misleading, or puzzling question posed as a problem to be solved or guessed : conundrum, enigma
something or someone difficult to understand
mysteryproblemenigmariddlepuzzle mean something which baffles or perplexes. mystery applies to what cannot be fully understood by reason or less strictly to whatever resists or defies explanation. problem applies to a question or difficulty calling for a solution or causing concern.