adj

• (military) Of a fortress or other fortified place: able to withstand all attacks; impenetrable, inconquerable, unvanquishable.
  "Unerringly impelling this dead, impregnable, uninjurable wall, and this most buoyant thing within; there swims behind it all a mass of tremendous life, only to be adequately estimated as piled wood is—by the cord; and all obedient to one volition, as the smallest insect."
• (figuratively) Too strong to be defeated or overcome; invincible.
  "[A]s for the Friendſhip of the World; […] he may at laſt be no more able to get into the other's Heart, than he is to thruſt his Hand into a Pillar of Braſs. The Man's Affection, amidſt all theſe Kindneſſes done him, remaining wholly unconcerned, and impregnable; juſt like a Rock, which being plied continually by the Waves, ſtill throws them back again into the Boſom of the Sea that ſent them, but is not at all moved by any of them."