sir

c. 1200, a title placed before a name and denoting knighthood, from Old French sire "lord (appellation), sire, my lord," from Vulgar Latin *seior, from Latin senior "older, elder" (from PIE root *sen- "old"). Later sir (q.v.), an alteration of sire, was used for this.

Wulcume sire Arður, wilcume lauerd.

[Laʒamon's "Brut," c. 1200]

Standing alone and meaning "your majesty" it is attested from early 13c. The general sense of "important elderly man" is from mid-14c.; that of "father, male parent, forefather" (as in grandsire) is from mid-13c., paired with dame. From 1520s as "male parent of a quadruped animal," especially a domestic animal, with dam (n.2) for the female parent.

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."