inexpugnable

from WordNet (r) 3.0 (2006)

inexpugnable
    adj 1: incapable of being overcome, challenged or refuted; "an
           impregnable argument"; "impregnable self-confidence"
           [syn: {impregnable}, {inexpugnable}]

from The Collaborative International Dictionary of English v.0.48

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.
   [1913 Webster]

         A fortress, inexpugnable by the arts of war. --Milman.
   [1913 Webster]