im ‧ preg ‧ na ‧ ble /ɪmˈpreɡnəb ə l/ BrE AmE adjective formal
[ Date: 1400-1500 ; Language: Old French ; Origin: imprenable 'impossible to capture' , from prendre 'to take' ]
1 . a building that is impregnable is so strong that it cannot be entered by force:
an impregnable fortress
2 . strong and impossible to change or influence:
her impregnable obstinacy