Is it ensure, insure, or assure?
1660s, "that which causes weariness," from French fatigue "weariness," from fatiguer "to tire," from Latin fatigare, originally "to cause to break down," later, "to weary, fatigue, tire out," from pre-Latin adj. *fati-agos "driving to the point of breakdown," from Old Latin *fatis (of unknown origin, related to adv. affatim "sufficiently" and to fatisci "crack, split") + root of agere "to drive" (see act (n.)). Meaning "weariness from exertion" is from 1719.
1690s, from French fatiguer (15c.), from fatigue (see fatigue (n.). Earlier in same sense was fatigate (1530s). Related: Fatigued; fatiguing.
fatigue fa·tigue (fə-tēg')
Physical or mental weariness resulting from exertion.
A sensation of boredom and lassitude due to absence of stimulation, to monotony, or to lack of interest in one's surroundings.
The decreased capacity or complete inability of an organism, an organ, or a part to function normally because of excessive stimulation or prolonged exertion.