The Collaborative International Dictionary of English v.0.48:

Inexpressible \In`ex*press"i*ble\, a.
   Not capable of expression or utterance in language;
   ineffable; unspeakable; indescribable; unutterable; as,
   inexpressible grief or pleasure. "Inexpressible grandeur."
   --Blair.
   [1913 Webster]

         In orbs
         Of circuit inexpressible they stood.     --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inexpressible
    adj 1: defying expression [syn: inexpressible,
           unexpressible] [ant: expressible]