"unassailable" in the adjective sense
1. impregnable, inviolable, secure, strong, unassailable, unattackable
immune to attack incapable of being tampered with
"an impregnable fortress"
"fortifications that made the frontier inviolable"
"a secure telephone connection"
2. unassailable, untouchable
impossible to assail
3. unassailable, unshakable, watertight, bulletproof
without flaws or loopholes
"an ironclad contract"
"a watertight alibi"
"a bulletproof argument"
Source: WordNet® (An amazing lexical database of English)
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