the French equivalent both of “sir” (in addressing a man directly) and of “mister,” or “Mr.” Etymologically it means “my lord” (mon sieur).

As an honorific title in the French royal court, it came to be used to refer to or address the eldest living brother of the king. The title Monsieur, without an adjoining proper name, was most notably first applied to Henry III's brother, François, duc d'Alençon, who by the Peace of Chastenoy (1576), popularly called the “Peace of Monsieur,” became the duc d'Anjou. The title was afterward assumed successively by Gaston, duc d'Orléans (brother of Louis XIII); Philippe, duc d'Orléans (brother of Louis XIV); Louis, comte de Provence (brother of Louis XVI; later himself King Louis XVIII); and Charles, comte d'Artois (brother of Louis XVIII; later himself King Charles X).