inexpugnable
Impossible to eliminate or destroy; impregnable.
My one hope of the world, my inexpugnable consolation in looking at the miseries of the world, is that this is altering.
Where it comes from
Learned borrowing from Latin inexpugnābilis. Morphologically, from in- + expugn + -able.
Adapted from the Wiktionary entry inexpugnable (CC BY-SA 4.0).