indefatigable

UK: /ˌɪndɪˈfætɪgəb^əl/

US: /ˌɪndɪˈfætɪgəbəl/

US: (in′di fat′i gə bəl)

WordReference Random House Learner's Dictionary of American English © 2025

in•de•fat•i•ga•ble /ˌɪndɪˈfætɪgəbəl/   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 © 2025

in•de•fat•i•ga•ble  (in′di fat′i gə bəl), adj. 

1. incapable of being tired out;
   not yielding to fatigue;
   untiring.

• 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•bil′i•ty, in′de•fat′i•ga•ble•ness, n. 
in′de•fat′i•ga•bly, adv. 
Synonyms

tireless, inexhaustible, persevering.

Collins Concise English Dictionary © HarperCollins Publishers::

indefatigable / ˌɪndɪˈfætɪɡəb^əl/ adjective

1. unable to be tired out; unflagging

Etymology: 16th Century: from Latin indēfatīgābilis, from in-¹ + dēfatīgāre, from fatīgāre to tire

ˌindeˌfatigaˈbility, ˌindeˈfatigableness noun

ˌindeˈfatigably adverb