Etymology
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indefatigable (adj.)

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.

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

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tireless (adj.)
1590s, "indefatigable," from tire (v.) + -less. From 1862 in the sense "without a tire," from tire (n.). Related: Tirelessly.
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