E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Latin, senex; Spanish, señor; Italian, signor; French, sieur; Norman, sire; English, sir. According to some, Greek is connected with Sir; on the analogy of ε- (ε) = Latin sum; εε = Latin semper; oo = Latin sapa.
Sir (a clerical address). Clergymen had at one time Sir prefixed to their name. This is not the Sir of knighthood, but merely a translation of the university word domnus given to graduates, as Dominus Hugh Evans, etc.