enigma (n.)

1530s, "statement which conceals a hidden meaning or known thing under obscure words or forms," earlier enigmate (mid-15c.), from Latin aenigma "riddle," from Greek ainigma (plural ainigmata) "a dark saying, riddle," from ainissesthai "speak obscurely, speak in riddles," from ainos "tale, story; saying, proverb;" according to Liddell & Scott, a poetic and Ionic word, of unknown origin. General sense in English of "anything inexplicable to an observer" is from c. 1600.