indefatigable
[in-di-fat-i-guh-buhl]
adjective
incapable of being tired out; not yielding to fatigue; untiring.
(dictionary.com; audio also available)
and m-w.com’s entry; audio also available:
Main Entry: in
de
fat
i
ga
ble
Pronunciation: \ˌin-di-ˈfa-ti-gə-bəl\
Function: adjective
Etymology: Middle French, from Latin indefatigabilis, from in- + defatigare to fatigue, from de- + fatigare to fatigue
Date: 1608
: incapable of being fatigued : untiring