In French, which word is used as a courtesy title of respect?

Monsieur (from Middle French mon sieur, literally "my lord") is an honorific title that was used to refer to or address the eldest living brother of the king in the French royal court. It has now become the customary French title of respect and term of address for a French-speaking man, corresponding to such English titles as Mr. or sir.

Hercule François, Duke of Anjou and Alençon (1555–1584), was the first notable member of the royalty to assume the title without the use of an adjoining proper name. In 1576, Monsieur pressured his brother King Henry III of France into signing the Edict of Beaulieu and effectively ending the Fifth Religious War of France. The resulting peace became popularly known as the Peace of Monsieur.

The title was later assumed by Gaston, Duke of Orléans, brother of Louis XIII, and then Philippe I, Duke of Orléans, brother of Louis XIV. From 1643 to 1660, while both princes were alive, Philippe was commonly known as le Petit Monsieur, while Gaston, his uncle, was known as le Grand Monsieur.

For over seventy years, from 1701 to 1774, the title had no living representatives in the French court, as Philippe of France, died in 1701; Louis XV was the youngest of the sons of Louis of France, Duke of Burgundy and at the time of his accession to the throne in 1715 had no brothers.

The title was restored in 1775 for Louis Stanislas Xavier, Count of Provence, the oldest surviving brother of the reigning Louis XVI and the future Louis XVIII.