Yours, Etc.: Origins and Uses of 8 Sign-Offs
c.1200, 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" (see senior (adj.)). Standing alone and meaning "your majesty" it is attested from early 13c. General sense of "important elderly man" is from mid-14c.; that of "father, male parent" is from mid-13c.
"to beget, to be the sire of," 1610s, from sire (n.). Used chiefly of beasts, especially of stallions. Related: Sired; siring.