Etymology
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Words related to indefatigable

in- (1)
Origin and meaning of in-
word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.
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de- 

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-.

fatigue (n.)
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.
defatigable (adj.)
Origin and meaning of defatigable

"liable to be wearied," 1650s, from defatigate (v.), 1550s, from Latin defatigatus, past participle of defatigare "to weary, tire out, exhaust with labor," from de "utterly, down, away" (see de-) + fatigare "to weary" (see fatigue (n.)). Also see indefatigable.

indefatigability (n.)
1630s, from indefatigable + -ity. Indefatigableness is from 1650s; indefatigation from 1640s.