unassailable

adjectives

immune to attack; incapable of being tampered with
"an impregnable fortress"; "fortifications that made the frontier inviolable"; "a secure telephone connection"
impregnable, inviolable, secure, strong, unassailable, unattackable
impossible to assail
unassailable, untouchable
without flaws or loopholes
"an ironclad contract"; "a watertight alibi"; "a bulletproof argument"
bulletproof, unassailable, unshakable, watertight