Source : Webster's Revised Unabridged Dictionary (1913)

Indefatigable \In`de*fat"i*ga*ble\, 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.

         Upborne with indefatigable wings.        --Milton.

   Syn: Unwearied; untiring; persevering; persistent.

Source : WordNet®

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