The Collaborative International Dictionary of English v.0.48:

Indefatigable \In`de*fat"i*ga*ble\
   ([i^]n`d[-e]*f[a^]t"[i^]*g[.a]*b'l), a. [L. indefatigabilis:
   cf. OF. indefatigable. See In- not, and Defatigable, and
   cf. Infatigable.]
   Incapable of being fatigued; not readily exhausted;
   unremitting in labor or effort; untiring; unwearying; not
   yielding to fatigue; as, indefatigable exertions,
   perseverance, application. "A constant, indefatigable
   attendance." --South.
   [1913 Webster]

         Upborne with indefatigable wings.        --Milton.

   Syn: Unwearied; untiring; persevering; persistent.
        [1913 Webster]

WordNet (r) 3.0 (2006):

indefatigable
    adj 1: showing sustained enthusiastic action with unflagging
           vitality; "an indefatigable advocate of equal rights"; "a
           tireless worker"; "unflagging pursuit of excellence"
           [syn: indefatigable, tireless, unflagging,
           unwearying]

Moby Thesaurus II by Grady Ward, 1.0:

88 Moby Thesaurus words for "indefatigable":
   abiding, ardent, assiduous, constant, continuing, continuous,
   determined, diligent, dogged, enduring, energetic, faithful,
   fervent, hard, hardworking, immutable, inalterable, indomitable,
   industrious, inexhaustible, insistent, invincible, laborious,
   lasting, loyal, never idle, never-tiring, obstinate, painstaking,
   patient, patient as Job, permanent, perseverant, persevering,
   persistent, persisting, pertinacious, plodding, plugging,
   preoccupied, rapt, relentless, remaining, resolute, sedulous,
   single-minded, sleepless, slogging, stable, staying, steadfast,
   steady, sticking, strenuous, stubborn, sustained, tenacious,
   tireless, unabating, unceasing, unconquerable, undaunted,
   undiscouraged, undrooping, undying, unfailing, unfaltering,
   unflagging, unflinching, unintermitting, uninterrupted, unnodding,
   unrelaxing, unrelenting, unremitting, unsleeping, unsparing,
   unswerving, untiring, unwavering, unwearied, unwearying, unwinking,
   utterly attentive, vehement, vigorous, weariless, zealous