(a.) Incapable of being spoken or voiced; inexpressible; ineffable; unspeakable; as, unutterable anguish\.

arcane awesome awful cabalistic censored classified close closed concealed cryptic dark divine enigmatic esoteric heavenly hermetic hidden holy hush-hush incommunicable inconceivable incredible indefinable indescribable ineffable inenarrable inexpressible innominable inviolable inviolate latent marvelous mysterious noncommunicable numinous occult prodigious religious restricted sacred sacrosanct secret smothered spiritual stifled suppressed top secret ulterior unbelievable unbreatheable undefinable under security under wraps undisclosable undisclosed undivulgable undivulged unimaginable unmentionable unnameable unrevealable unrevealed unspeakable unspoken untellable untold untouchable unuttered unwhisperable venerable wonderful wondrous