【Indefatigable】 , 【tireless】 ,  【weariless】 ,  【untiring】 ,  【unwearying】 ,  【unwearied】 ,  【unflagging】  are comparable in their basic meaning of not feeling or manifesting fatigue, but they are closer synonyms in their extended sense of capable of prolonged and arduous effort.

【Indefatigable】 implies being incapable of being fatigued, but in its actual use it usually suggests persistent and unremitting activity or effort.

【Tireless】  and  【weariless】  are sometimes employed with little distinction from  【indefatigable】 ; frequently, however, they connote less busyness and even greater or more remarkable power of continuance.

【Untiring】  and  【unwearying】  differ from  【tireless】  in carrying a stronger implication of uninterrupted activity; often they specifically suggest an extraordinary ability to go on continuously and without a break while  【tireless】  and  【weariless】 , by contrast, often imply repeated returns over a very long course of time.

【Unwearied】  differs little from  【untiring】 in its meaning, but it is more often applied directly to the person or thing concerned than to the activity engaged in.

【Unflagging】  differs little from  【tireless】 , for it too stresses a display of power to continue without signs of weariness; but it also stresses no diminution of activity, and it applies to a person’s powers rather than to the person himself.