"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.