definition of
[impregnable]

[1] #1 impregnable s. [inviolable, secure, strong, unassailable, unattackable] immune to attack; incapable of being tampered with; "an impregnable fortress"; "fortifications that made the frontier inviolable"; "a secure telephone connection". wn

[2] #2 impregnable s. [conceptive] capable of conceiving. wn

[3] #3 impregnable s. [inexpugnable] incapable of being overcome, challenged or refuted; "an impregnable argument"; "impregnable self-confidence". wn