Definition of indefatigable adjective from the Oxford Advanced Learner's Dictionary

 

indefatigable

 adjective
adjective
BrE BrE//ˌɪndɪˈfætɪɡəbl//
 
; NAmE NAmE//ˌɪndɪˈfætɪɡəbl//
 
(formal, approving)
 
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never giving up or getting tired of doing something an indefatigable defender of human rights She was indefatigable in her search for the truth. Word Origin early 17th cent.: from French, or from Latin indefatigabilis, from in- ‘not’ + de- ‘away, completely’ + fatigare ‘wear out’.
See the Oxford Advanced American Dictionary entry: indefatigable