Source : Webster's Revised Unabridged Dictionary (1913)

Indescribable \In`de*scrib"a*ble\, a.
   Incapable of being described. -- {In`de*scrib"a*bly}, adv.

Source : WordNet®

indescribably
     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}, {unutterably}, {unspeakably}]