an impregnable building is so strongly built that it cannot be entered by force 坚不可摧的；牢不可破的 an impregnable fortress 坚不可摧的要塞
strong and impossible to defeat or change 不可战胜的；难以改变的
Word Originlate Middle English: from Old French imprenable, from in- ‘not’ + prendre ‘take’ (from Latin prehendere). The current spelling arose in the 16th cent., perhaps influenced by Old French variants.