Roget's Thesaurus of English Words and Phrases by Peter Mark Roget

533. Secret — N. secret; dead secret, profound secret; arcanum†,

mystery; latency &c 526; Asian mystery†; sealed book, secrets of the prison house; le desous des cartes [Fr.]. enigma, riddle, puzzle, nut to crack, conundrum, charade, rebus, logogriph†; monogram, anagram; Sphinx; crux criticorum [Lat.]. maze, labyrinth, Hyrcynian wood; intricacy, meander. problem &c (question) 461; paradox &c (difficulty) 704; unintelligibility &c 519; terra incognita &c (ignorance) 491. Adj. secret &c (concealed) 528; involved &c, 248; labyrinthine, labyrinthian†, mazy. confidential; top secret.