Source : Webster's Revised Unabridged Dictionary (1913)

Unutterable \Un*ut"ter*a*ble\, a.
   Not utterable; incapable of being spoken or voiced;
   inexpressible; ineffable; unspeakable; as, unutterable
   anguish.

         Sighed and looked unutterable things.    --Thomson.
   -- {Un*ut"ter*a*ble*ness}, n. -- {Un*ut"ter*a*bly}, adv.

Source : WordNet®

unutterable
     adj 1: too sacred to be uttered; "the ineffable name of the Deity"
            [syn: {ineffable}, {unnameable}, {unspeakable}]
     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}]
     3: very difficult to pronounce correctly; "an unpronounceable
        foreign word"; "unutterable consonant clusters" [syn: {unpronounceable}]