1. 攻不破的：good penny worth 便宜货 | unassailable 攻不破的 | upper ionized layer 高电离层

2. 不容置疑的：unapt 不合适的 | unassailable 不容置疑的 | unassertive 谦逊的

3. 无可反驳的：unasked 未被要求的 | unassailable 无可反驳的 | unassertive 谦虚的

4. 无懈可击 無懈可擊：无与伦比 無與倫比 without equal | 无懈可击 無懈可擊 unassailable | 无影无踪 無影無蹤 vanish or disappear without a trace

1. 坚不可摧的；无懈可击的
    If you describe something or someone as unassailable, you mean that nothing can alter, destroy, or challenge them.

    e.g. That was enough to give Mansell an unassailable lead...
           那足以让曼塞尔获得别人无法撼动的领先地位。
    e.g. His legal position is unassailable...
           他的合法地位是不容置疑的。

1. without flaws or loopholes

    e.g. an ironclad contract
           a watertight alibi
           a bulletproof argument

    Synonym: unshakablewatertightbulletproof

2. impossible to assail

    Synonym: untouchable

3. immune to attack
    incapable of being tampered with

    e.g. an impregnable fortress
           fortifications that made the frontier inviolable
           a secure telephone connection

    Synonym: impregnableinviolablesecurestrongunattackable