late 15c., "one who acts," from Latin agentem (nominative agens) "effective, powerful," present participle of agere "to set in motion, drive forward; to do, perform; keep in movement" (from PIE root *ag- "to drive, draw out or forth, move"). Meaning "any natural force or substance which produces a phenomenon" is from 1550s. Meaning "deputy, representative" is from 1590s. Sense of "spy, secret agent" is attested by 1916.
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