impregnable

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.

Jurgis got up, wild with rage; but the door was shut and the great castle was dark and impregnable.

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.

And with Bolton suffering a wretched run of five straight home defeats – their worst run in 109 years – Chelsea fans would have been forgiven for expecting a comfortable win. But surely they did not anticipate the ease with which their team raced into an almost impregnable half-time lead.

Capable of being impregnated; impregnatable.

The reproductive strategies of troop members, especially those of impregnable females, are suggested to influence patterns of range use.