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