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”