WordReference Random House Learner's Dictionary of American English © 2015
in•de•fat•i•ga•ble /ˌɪndɪˈfætɪgəbəl/USA pronunciation   adj. 
    1. incapable of being tired out:an indefatigable worker.
in•de•fat•i•ga•bly, adv.: worked indefatigably.

WordReference Random House Unabridged Dictionary of American English © 2015
in•de•fat•i•ga•ble  (in′di fati gə bəl), 
    1. incapable of being tired out;
      not yielding to fatigue;
  • Latin indēfatīgābilis untiring, equivalent. to in- in-3 + dēfatīgā(re) to tire out (see de-, fatigue) + -bilis -ble
  • 1580–90
in′de•fat′i•ga•bili•ty, in′de•fati•ga•ble•ness, n. 
in′de•fati•ga•bly, adv. 
tireless, inexhaustible, persevering.

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

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