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]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "indomitable":
   assiduous, beyond control, brave, breachy, constant, continuing,
   contumacious, courageous, dauntless, defiant, determined, diligent,
   dogged, enduring, faithful, fearless, fractious, immutable,
   impassable, impenetrable, impregnable, inalterable, incontestable,
   incorrigible, indefatigable, indocile, industrious, inexpugnable,
   insistent, insuperable, insuppressible, insurmountable,
   intractable, intrepid, invincible, inviolable, invulnerable,
   irrepressible, irresistible, lasting, loyal, mettlesome,
   never-tiring, obstinate, obstreperous, out of hand, overpowering,
   overwhelming, patient, patient as Job, permanent, perseverant,
   persevering, persistent, persisting, pertinacious, plodding,
   plucky, plugging, preoccupied, rapt, recalcitrant, refractory,
   relentless, resistant, resisting, resistless, resolute, resolved,
   restive, sedulous, shrewish, single-minded, sleepless, slogging,
   stable, staunch, steadfast, steady, stubborn, tenacious, tireless,
   unabating, unafraid, unassailable, unbeatable, unbiddable,
   unconquerable, uncontrollable, undaunted, undefeatable,
   undisciplined, undiscouraged, undrooping, unfailing, unfaltering,
   unflagging, unflinching, ungovernable, unintermitting,
   uninterrupted, unmalleable, unmanageable, unmoldable, unnodding,
   unrelaxing, unrelenting, unremitting, unruly, unsleeping,
   unstoppable, unsubduable, unsubmissive, unsurmountable, unswerving,
   untiring, unwavering, unwearied, unwearying, unwinking, unyielding,
   utterly attentive, weariless, wild