mystery

The word 'mystery' entered English in the early 14th century from the Anglo-French 'misterie' and Old French 'mistere', meaning a secret or hidden truth. These derive from the Latin 'mysterium', which referred to a secret rite or doctrine, particularly in religious contexts. The Latin term itself originates from the Greek 'mystērion', meaning a secret rite or doctrine, related to 'mystēs', an initiate into religious secrets, which comes from the verb 'myein', to close (the eyes or lips), emphasizing the concept of secrecy and things kept from open knowledge. The semantic journey thus flows from the literal act of closing or concealing, to sacred secrets known only to initiates, and finally to its modern senses of anything inexplicable or a genre of fiction centered on solving a concealed crime.

The word is a simplex loanword from French. It cannot be meaningfully segmented into productive morphemes in modern English spelling without resorting to etymological forms not present in the word (e.g., 'myster' + 'y' is not a standard analysis as '-y' is not a clear suffix here, and 'myster' is not a free morpheme).