unassailable
un-as-sail-a-ble
adjective
Impossible to attack, defeat, or question; completely secure and strong.
"His argument was unassailable, leaving no room for doubt."
Thesaurus
Synonyms: inviolable, untouchable, impregnable, strong, invulnerable, unattackable, secure, ironclad, incontestable, bulletproof, unshakable, watertight