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
impregnable 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.