THESAURUS
arcane, cabalistic, censored, classified, close, closed, concealed, cryptic, dark, enigmatic, esoteric, hermetic, hidden, hush-hush, latent, mysterious, occult, restricted, secret, smothered, stifled, suppressed, ulterior, unbreatheable, under security, under wraps, undisclosable, undisclosed, undivulgable, undivulged, unrevealable, unrevealed, unspoken, untellable, untold, unutterable, unuttered, unwhisperable
ROGET THESAURUS
Secret
N
secret, dead secret, profound secret, arcanum, mystery, latency, Asian mystery, sealed book, secrets of the prison house, le desous des cartes, enigma, riddle, puzzle, nut to crack, conundrum, charade, rebus, logogriph, monogram, anagram, Sphinx, crux criticorum, maze, labyrinth, Hyrcynian wood, intricacy, meander, problem, paradox, unintelligibility, terra incognita, secret, involved, labyrinthine, labyrinthian, mazy, confidential,
top secret.