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]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "inexpressible":
   awesome, awful, divine, heavenly, holy, incommunicable,
   indefinable, indescribable, ineffable, inenarrable, innominable,
   inviolable, inviolate, noncommunicable, numinous, religious,
   sacred, sacrosanct, spiritual, undefinable, unmentionable,
   unnameable, unspeakable, untellable, untouchable, unutterable,
   unwhisperable, venerable