impregnable
/ɪmˈpregnəbl; ɪm`prɛɡnəbl/ adj
(a) so strong and well-constructed that it cannot be entered orcaptured 坚固得不能进入或无法占领的
an impregnable fortress 坚不可摧的要塞.
(b) (fig 比喻) so strong that it cannot be overcome or broken down 无法征服的; 压不倒摧不垮的
impregnable arguments, defences, reserve 驳不倒的论点、 有理有据的辩护、 无法改变的拘谨.