impregnable

(s) immune to attack; incapable of being tampered with
- an impregnable fortress
- fortifications that made the frontier inviolable
- a secure telephone connection
unassailable, unattackable, inviolable, secure, strong,
(s) capable of conceiving conceptive,
(s) incapable of being overcome, challenged or refuted
- an impregnable argument
- impregnable self-confidence
inexpugnable,

unassailable unattackable inviolable secure strong conceptive inexpugnable

impregnably