immune to attack; incapable of being tampered with; "an impregnable fortress"; "fortifications that made the frontier inviolable"; "a secure telephone connection"攻撃に対しての免疫がある頑強
例:an impregnable fortress 難攻不落の要塞
incapable of being overcome, challenged or refuted; "an impregnable argument"; "impregnable self-confidence"克服される、挑戦ざれる、または反駁される険要