closed book

closed book

noun

Definition 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