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 ]