impregnable

adjectives

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