1. (adj)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. (adj)unassailable, untouchable impossible to assail
3. (adj)unassailable, unshakable, watertight, bulletproof without flaws or loopholes
"an ironclad contract"; "a watertight alibi"; "a bulletproof argument"
Definition of 'unassailable'
