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adjective
hidden, unrevealed
Synonyms: abstruse, ambiguous, arcane, backdoor, camouflaged, classified, cloak-and-dagger, close, closet, clouded, conspiratorial, covered, covert, cryptic, dark, deep, disguised, enigmatical, esoteric, furtive, hush-hush, mysterious, mystic, mystical, obscure, occult, on the QT, out-of-the-way, private, recondite, reticent, retired, secluded, shrouded, strange, undercover, underground, under wraps, undisclosed, unenlightened, unfrequented, unintelligible, unknown, unpublished, unseen, veiled
Antonyms: clear, defined, explicit, forthright, honest, open, public, revealed, unconcealed
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adjective
underhand, clandestine
Synonyms: backdoor, backstairs, camouflaged, classified, close, confidential, covert, cryptic, discreet, disguised, dissembled, dissimulated, furtive, hush-hush, in ambush, incognito, inside, restricted, secretive, sly, sneak, sneaky, stealthy, sub-rosa, surreptitious, top secret, unacknowledged, under false pretense, underhanded, under-the-table
Antonyms: aboveboard, known, legal, legitimate, public
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noun
something kept hidden, unrevealed
Synonyms: cipher, classified information, code, confidence, confidential information, enigma, formula, key, magic number, mystery, occult, oracle, password, privileged information, puzzle, skeleton in cupboard, unknown
Antonyms: known, public knowledge, revelation