Source : Webster's Revised Unabridged Dictionary (1913)

Inexpugnable \In`ex*pug"na*ble\, a. [L. inexpugnabilis: cf. F.
   inexpugnable. See {In-} not, and {Expugnable}.]
   Incapable of being subdued by force; impregnable;
   unconquerable. --Burke.

         A fortress, inexpugnable by the arts of war. --Milman.

Source : WordNet®

inexpugnable
     adj : impossible to take by storm [syn: {impregnable}]