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").
The meaning "any natural force or substance which produces a phenomenon" is from 1550s. The meaning "deputy, representative" is from 1590s. The sense of "spy, secret agent" is attested by 1916.
updated on September 15, 2022