1. 
[syn: indefinable, indescribable, ineffable, unspeakable, untellable, unutterable]
The Collaborative International Dictionary of English v.0.48:
Indescribable \In`de*scrib"a*ble\, a.
   Incapable of being described.
   [1913 Webster]
WordNet (r) 3.0 (2006):
indescribable
    adj 1: defying expression or description; "indefinable
           yearnings"; "indescribable beauty"; "ineffable ecstasy";
           "inexpressible anguish"; "unspeakable happiness";
           "unutterable contempt"; "a thing of untellable splendor"
           [syn: indefinable, indescribable, ineffable,
           unspeakable, untellable, unutterable]
Moby Thesaurus II by Grady Ward, 1.0:
31 Moby Thesaurus words for "indescribable":
   exceptional, extraordinary, fabulous, incommunicable, indefinable,
   ineffable, inenarrable, inexpressible, innominable, legendary,
   marvelous, mythical, noncommunicable, noteworthy, phenomenal,
   portentous, prodigious, remarkable, stupefying, stupendous,
   undefinable, unexampled, unmentionable, unnameable, unparalleled,
   unprecedented, unspeakable, untellable, unutterable, unwhisperable,
   wonderful