That which can not be believed - incredible (अविश्वसनीय)
That which can not be conquered - invincible (अजेय)
That which can not be cured - incurable (लाइलाज)
That which can not be defended - indefensible (अरक्षणीय)
That which can not be described - indescribable (अवर्णनीय)