From Longman Dictionary of Contemporary Englishimpregnableim‧preg‧na‧ble /ɪmˈpreɡnəbəl/ adjective formal 1 DEFENDa building that is impregnable is so strong that it cannot be entered by force an impregnable fortress2 STUBBORNstrong and impossible to change or influence her impregnable obstinacy
Examples from the Corpus
impregnable• The conventional wisdom having been made more or less identical with sound scholarship, its position is virtually impregnable.• The one factor that still tilts general election predictions in the Tories' favour is that their hillcrest position seems ultimately impregnable.• The problem invariably is that the enemy is simply inflexible or impregnable.• The case Starr builds must be as impregnable as Fort Knox.• And Andrus would be untouched, impregnable behind his rigid simplicities.• Occupying fairly impregnable clifftop villages, they prospered in the practice of agriculture.• Subject, verb, object: the unadorned, impregnable sentence.• Grant was still mired in the mud before impregnable Vicksburg.Origin impregnable (1400-1500) Old French imprenable “impossible to capture”, from prendre “to take” 