defatigable (adj.)

active word-forming element in English and in many verbs inherited from French and Latin, from Latin de "down, down from, from, off; concerning" (see de), also used as a prefix in Latin, usually meaning "down, off, away, from among, down from," but also "down to the bottom, totally" hence "completely" (intensive or completive), which is its sense in many English words.

As a Latin prefix it also had the function of undoing or reversing a verb's action, and hence it came to be used as a pure privative — "not, do the opposite of, undo" — which is its primary function as a living prefix in English, as in defrost (1895), defuse (1943), de-escalate (1964), etc. In some cases, a reduced form of dis-.

1660s, "that which causes weariness," from French fatigue "weariness," from fatiguer "to tire" (15c.), from Latin fatigare "to weary, to tire out," originally "to cause to break down," from pre-Latin adjective *fati-agos "driving to the point of breakdown," with first half from Old Latin *fatis, which is of unknown origin but apparently related to affatim (adv.) "sufficiently" and to fatisci "crack, split." The second half is the root of agere "to set in motion, drive; to do, perform" (from PIE root *ag- "to drive, draw out or forth, move").

Especially "the labors of military persons" (1776). Meaning "a feeling of weariness from exertion" is from 1719. Of metals or other materials under strain, from 1877.

1580s (implied in indefatigably), from French indefatigable (15c.), from Latin indefatigabilis "that cannot be wearied," from in- "not" (see in- (1)) + defatigare "to tire out," from de- "utterly, down, away" (see de-) + fatigare "to weary" (see fatigue (n.)).

Blount's "Glossographia" (1656) has defatigable, which also was in use elsewhere in 17c., but the modern use of defatigable (1948) probably is a jocular back-formation from indefatigable.