1. 不能攻占的：inexpressive 缺乏表情的 | inexpugnable 不能攻占的 | inexpungibility 不能除去

2. 不能攻占的/不能驳倒的：inexpressiveness /无表情/ | inexpugnable /不能攻占的/不能驳倒的/ | inextensibility /不可伸长性/非延伸性/

adj

1. incapable of being overcome, challenged or refuted

e.g. an impregnable argument
impregnable self-confidence

Synonym: impregnable