Monsieur

google

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French, from mon ‘my’ + sieur ‘lord’.

wiktionary

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French monsieur

etymonline

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monsieur (n.)

the common title of courtesy in France, equivalent to English mister, 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.