WordReference Random House Learner's Dictionary of American English © 2015
in•de•fat•i•ga•ble /ˌɪndɪˈfætɪgəbəl/USA pronunciation adj. 

    incapable of being tired out:an indefatigable worker.
in•de•fat•i•ga•bly, adv.: worked indefatigably.

Collins Concise English Dictionary © HarperCollins Publishers::

indefatigable /ˌɪndɪˈfætɪɡəbəl/ adj
  1. unable to be tired out; unflagging
Etymology: 16th Century: from Latin indēfatīgābilis, from in-1 + dēfatīgāre, from fatīgāre to tire

ˌindeˌfatigaˈbility, ˌindeˈfatigableness n ˌindeˈfatigably adv

'indefatigable' also found in these entries:

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