[syn: unpronounceable, unutterable]
The Collaborative International Dictionary of English v.0.48:
Unutterable \Un*ut"ter*a*ble\, a.
   Not utterable; incapable of being spoken or voiced;
   inexpressible; ineffable; unspeakable; as, unutterable
   anguish.
   [1913 Webster]
         Sighed and looked unutterable things.    --Thomson.
   [1913 Webster] -- Un*ut"ter*a*ble*ness, n. --
   Un*ut"ter*a*bly, adv.
   [1913 Webster]
WordNet (r) 3.0 (2006):
unutterable
    adj 1: too sacred to be uttered; "the ineffable name of the
           Deity" [syn: ineffable, unnameable, unspeakable,
           unutterable]
    2: defying expression or description; "indefinable yearnings";
       "indescribable beauty"; "ineffable ecstasy"; "inexpressible
       anguish"; "unspeakable happiness"; "unutterable contempt"; "a
       thing of untellable splendor" [syn: indefinable,
       indescribable, ineffable, unspeakable, untellable,
       unutterable]
    3: very difficult to pronounce correctly; "an unpronounceable
       foreign word"; "unutterable consonant clusters" [syn:
       unpronounceable, unutterable]
Moby Thesaurus II by Grady Ward, 1.0:
72 Moby Thesaurus words for "unutterable":
   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