Source : Webster's Revised Unabridged Dictionary (1913)

Tireless \Tire"less\, a.
   Untiring.

Source : WordNet®

tireless
     adj 1: showing sustained enthusiastic action with unflagging
            vitality; "an indefatigable advocate of equal rights";
            "a tireless worker"; "unflagging pursuit of
            excellence" [syn: {indefatigable}, {unflagging}, {unwearying}]
     2: characterized by hard work and perseverance [syn: {hardworking},
         {industrious}, {untiring}]