Etymology
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monseigneur (n.)

French title of honor given to princes, bishops, and other dignitaries of the church or court, equivalent to my lord, c. 1600, from French monseigneur (12c.), from mon "my" (from Latin meum) + seigneur "lord," from Latin seniorem, accusative of senior "older" (from PIE root *sen- "old"). Plural messeigneurs.

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monsignor (n.)

title conferred on some prelates and dignitaries of the papal court and household, 1640s, from Italian monsignore "my lord," formed on model of French monseigneur (see monseigneur) from equivalent elements in Italian.

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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.

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*sen- 
Proto-Indo-European root meaning "old."

It forms all or part of: monseigneur; seignior; senate; senescent; seneschal; senicide; senile; senility; senior; seniority; senor; senora; senorita; shanachie; Shannon; signor; sir; sire; surly.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sanah "old;" Avestan hana- "old," Old Persian hanata- "old age, lapse of time;" Armenian hin "old;" Greek enos "old, of last year;" Latin senilis "of old age," senex "old, old man;" Lithuanian senas "old," senis "an old man;" Gothic sineigs "old" (used only of persons), sinistra "elder, senior;" Old Norse sina "dry standing grass from the previous year;" Old Irish sen, Old Welsh hen "old."
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