late 14c., "government, rule, control," from Old French regiment "government, rule" (14c.), from Late Latin regimentum "rule, direction," from Latin regere "to rule, to direct, keep straight, guide" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule"). Meaning "unit of an army" first recorded 1570s (originally the reference was to permanent organization and discipline), from French. The exact number in the unit varies over time and place.
"to form into a regiment," 1610s, from regiment (n.). General sense of "organize systematically" is from 1690s. Related: Regimented; regimenting.