Mawson, C.O.S., ed. (1870–1938). Roget’s International Thesaurus. 1922.

Class IV. Words Relating to the Intellectual Faculties
Division (II) Communication of Ideas
Section II. Modes of Communication

533. Secret.

   NOUN:SECRET; dead -, profound- secret; arcanum, mystery; latency [See Latency]; Asian mystery; sealed book, secrets of the prison house; le dessous des cartes [F.].
  ENIGMA, riddle, puzzle, nut to crack, conundrum, charade, rebus, logogriph; monogram, anagram, anagrammatism; Sphinx; crux criticorum [L.].
  MAZE, labyrinth, meander [usually in pl.], Hyrcynian wood; intricacy.
  PROBLEM (question) [See Inquiry]; paradox (difficulty) [See Difficulty]; unintelligibility [See Unintelligibility]; terra incognita [L.] (ignorance) [See Ignorance].
   ADJECTIVE:SECRET (concealed) [See Concealment]; involved [See Convolution]; labyrinthian, labyrinthine, labyrinthic or labyrinthical, mazy, meandrous.
  ENIGMATIC or enigmatical, anagrammatic or anagrammatical, monogrammatic, logogriphic, cryptic or cryptical.
   QUOTATION:She was more mystical than Woman.—Dunsany