impregnable inviolable secure strong unassailable unattackable
impregnable%5:00:00:invulnerable:00 inviolable%5:00:00:invulnerable:00 secure%5:00:00:invulnerable:00 strong%5:00:00:invulnerable:00 unassailable%5:00:00:invulnerable:00 unattackable%5:00:00:invulnerable:00
immune to attack; incapable of being tampered with
an impregnable fortress fortifications that made the frontier inviolable a secure telephone connection