ancien regime (n.)

late 14c., auncyen, of persons, "very old;" c. 1400, of things, "having lasted from a remote period," from Old French ancien "old, long-standing, ancient," from Vulgar Latin *anteanus, literally "from before," adjectivization of Latin ante "before, in front of, against" (from PIE *anti "against," locative singular of root *ant- "front, forehead"). The unetymological -t dates from 15c. by influence of words in -ent.

From early 15c. as "existing or occurring in times long past." Specifically, in history, "belonging to the period before the fall of the Western Roman Empire" (c. 1600, contrasted with medieval and modern). In English law, "from before the Norman Conquest."

As a noun, "very old person," late 14c.; "one who lived in former ages," 1530s. Ancient of Days "supreme being" is from Daniel vii.9. Related: Anciently.

"system of government or rule, mode of management," 1792, from French régime, from Old French regimen (14c.), from Latin regimen "rule, guidance, government, means of guidance, rudder," from regere "to direct, to guide" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule").

Earlier "course of diet, exercise" (late 15c.), a sense now pertaining to regimen (q.v.). In French, l'ancien régime refers to the system of government which prevailed before the revolution of 1789.