unutterable in WordNet English dictionary

1. defying expression or description
   "indefinable yearnings"; "indescribable beauty"; "ineffable ecstasy"; "inexpressible anguish"; "unspeakable happiness"; "unutterable contempt"; "a thing of untellable splendor"
   indefinable  indescribable  ineffable  unspeakable  untellable  unutterable 
   
2. too sacred to be uttered
   "the ineffable name of the Deity"
   ineffable  unnameable  unspeakable  unutterable 
   
3. very difficult to pronounce correctly
   "an unpronounceable foreign word"; "unutterable consonant clusters"
   unpronounceable  unutterable