1510s, from French monsieur , from mon sieur "my lord," from sieur "lord," shortened form of seigneur (see monseigneur ) It was the historical title for the second son or next younger brother of the king of France.
Monsieur ( French pronunciation: [mə.sjø] ; pl. Messieurs ; 1512, from Middle French mon sieur , literally "my lord"  ) is an honorific title that 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 . 
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