unutterable

too sacred to be uttered; "the ineffable name of the Deity"

defying expression or description; "indefinable yearnings"; "indescribable beauty"; "ineffable ecstasy"; "inexpressible anguish"; "unspeakable happiness"; "unutterable contempt"; "a thing of untellable splendor"

very difficult to pronounce correctly; "an unpronounceable foreign word"; "unutterable consonant clusters"