Synonyms and related words :
arcane, cabalistic, censored, classified, close, closed, concealed, cryptic, dark, echoless, enigmatic, esoteric, hermetic, hidden, hush as death, hush-hush, hushed, implicit, implied, inaudible, inferred, latent, mousy, mysterious, noiseless, occult, quiescent, quiet, restricted, secret, silent, smothered, soundless, stifled, still, still as death, stilly, subaudible, suppressed, tacit, top secret, ulterior, unarticulated, unbreatheable, undeclared, under security, under wraps, understood, undisclosable, undisclosed, undivulgable, undivulged, unexpressed, unhearable, unmentioned, unproclaimed, unpronounced, unpublished, unrecorded, unrevealable, unrevealed, unsaid, unsounded, unspoken, unsung, untalked-of, untellable, untold, unutterable, unvocalized, unvoiced, unwhisperable, unwritten, whist, wordless