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®

unutterably
     adv : to an inexpressible degree; "she was looking very young
           tonight, and, as usual, indescribably beautiful, in a
           simple strapless dress of a green and white silky
           cotton" [syn: {ineffably}, {indescribably}, {unspeakably}]