Thesaurus
closed book
noun
Synonyms and Antonyms of CLOSED BOOK
something hard to understand or explain <the couple's unorthodox marriage is definitely a closed book to those outside of the relationship>
Synonyms closed book, conundrum, enigma, head-scratcher, mystification, puzzle, puzzlement, riddle, secret
Related Words brainteaser, case, challenge, knot, matter, perplexity, poser, problem, stumper, trouble
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