1. 
[syn: ineffably, indescribably, unutterably, unspeakably]
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):
unutterably
    adv 1: 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, unutterably,
           unspeakably]