indefatigable Look up indefatigable at Dictionary.com
1586 (implied in indefatigably), from Fr. indefatigable (15c.), from L. indefatigabilis "that cannot be wearied," from in- "not" + defatigare "to tire out," from de- "utterly, down, away" + fatigare "to weary" (see fatigue).