1400-1500Old Frenchimprenable ‘impossible to capture’, from prendre ‘to take’
Examples
EXAMPLES FROM THE CORPUS
And Andrus would be untouched, impregnable behind his rigid simplicities.
Grant was still mired in the mud before impregnable Vicksburg.
Occupying fairly impregnable clifftop villages, they prospered in the practice of agriculture.
Subject, verb, object: the unadorned, impregnable sentence.
The case Starr builds must be as impregnable as Fort Knox.
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.
1a building that is impregnable is so strong that it cannot be entered by force: an impregnable fortress2strong and impossible to change or influence: her impregnable obstinacy