inexpugnable
inex·pug·nable
adjective
1.
impossible to take by storm
Origin
Learned borrowing from Latin inexpugnābilis. Morphologically, from in- + expugn + -able.
WordNet 2.1 (Princeton University). Wiktionary (CC BY-SA). CMU Pronouncing Dictionary. WordNet 2.1 Copyright Princeton University. All rights reserved. Wiktionary content is available under the Creative Commons Attribution-ShareAlike Licence.