im·preg·na·ble
/imˈpreɡnəb(ə)l/
adjective: impregnable
(of a fortified position) unable to be captured or broken into.
- unable to be defeated or destroyed; unassailable.
Origin
late Middle English: from Old French imprenable, from in- ‘not’ + prendre ‘take’ (from Latin prehendere ). The current spelling arose in the 16th century, perhaps influenced by Old French variants.