Proto-Indo-European root meaning "to drive, draw out or forth, move."

It forms all or part of: act; action; active; actor; actual; actuary; actuate; agency; agenda; agent; agile; agitation; agony; ambagious; ambassador; ambiguous; anagogical; antagonize; apagoge; assay; Auriga; auto-da-fe; axiom; cache; castigate; coagulate; cogent; cogitation; counteract; demagogue; embassy; epact; essay; exact; exacta; examine; exigency; exiguous; fumigation; glucagon; hypnagogic; interact; intransigent; isagoge; litigate; litigation; mitigate; mystagogue; navigate; objurgate; pedagogue; plutogogue; prodigal; protagonist; purge; react; redact; retroactive; squat; strategy; synagogue; transact; transaction; variegate.

It is the hypothetical source of/evidence for its existence is provided by: Greek agein "to lead, guide, drive, carry off," agon "assembly, contest in the games," agōgos "leader," axios "worth, worthy, weighing as much;" Sanskrit ajati "drives," ajirah "moving, active;" Latin actus "a doing; a driving, impulse, a setting in motion; a part in a play;" agere "to set in motion, drive, drive forward," hence "to do, perform," agilis "nimble, quick;" Old Norse aka "to drive;" Middle Irish ag "battle."

"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.

1776, "extra duties of a soldier," from fatigue (n.). As a military clothing outfit, from 1836, short for fatigue dress (1833); fatigue cap is from 1824.

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.

"untiring," c. 1500, from French infatigable (15c.) or directly from Late Latin infatigabilis "that cannot be wearied," from in- "not, opposite of" (see in- (1)) + fatigabilis "that may be wearied," from Latin fatigare "to weary" (see fatigue (n.)).