in ‧ de ‧ fat ‧ i ‧ ga ‧ ble /ˌɪndɪˈfætɪɡəb ə l/ BrE AmE adjective formal
[ Date: 1600-1700 ; Origin: Early French indéfatigable , from Latin indefatigabilis , from fatigare ; ⇨ ↑ fatigue ]
determined and never giving up SYN tireless :
an indefatigable campaigner for human rights
—indefatigably adverb