incapable of being taken by assault
an impregnable fortress
beyond criticism or question
an impregnable social position
strong or; firm; unbeatable; unassailable
an impregnable belief in his own superiority
He has an impregnable belief in his own ability
impregnability /-ʹbiliti/ noun
impregnably adv
[Middle English imprenable from early French, from in-1 + prenable vulnerable to capture, from prendre to take, from Latin prehendere: see prehensile]