Source : Webster's Revised Unabridged Dictionary (1913)

Indomitable \In*dom"i*ta*ble\, a. [L. indomitabilis; pref. in-
   not + domitare, intens. fr. domare to tame. See {Tame}.]
   Not to be subdued; untamable; invincible; as, an indomitable
   will, courage, animal.

Source : WordNet®

indomitable
     adj : impossible to subdue [syn: {never-say-die}, {unsubduable}]