The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

indomitable
    adj 1: impossible to subdue [syn: indomitable, never-say-
           die, unsubduable]