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]