The usefulness of the Online Etymological Dictionary

is manifest once again?

From the Online Etymological Dictionary:

sire

c.1205, title placed before a name and denoting knighthood, from O.Fr. sire, from V.L. *seior, from L. senior "older, elder" (see senior). Standing alone and meaning "your majesty" it is attested from c.1225. General sense of "important elderly man" is from 1362; that of "father, male parent" is from c.1250. The verb meaning "to beget, to be the sire of" is attested from 1611, from the noun.

sir

1297, title of honor of a knight or baronet (until 17c. also a title of priests), variant of sire, originally used only in unstressed position. Generalized as a respectful form of address by c.1350; used as a salutation at the beginning of letters from 1425.

mister

as a title of courtesy before a man's Christian name, 1447, unaccented variant of master.

master (n.)

O.E. mægester "one having control or authority," from L. magister "chief, head, director, teacher" (cf. O.Fr. maistre, Fr. maître, It. maestro, Ger. Meister), infl. in M.E. by O.Fr. maistre, from L. magister, contrastive adj. from magis (adv.) "more," itself a comp. of magnus "great." Meaning "original of a recording" is from 1904. In academic senses (from M.L. magister) it is attested from 1380s, originally a degree conveying authority to teach in the universities. The verb is attested from c.1225.

lord

M.E. laverd, loverd (13c.), from O.E. hlaford "master of a household, ruler, superior," also "God" (translating L. Dominus, though O.E. drihten was used more often), earlier hlafweard, lit. "one who guards the loaves," from hlaf "bread, loaf" + weard "keeper, guardian, ward." Cf. lady, and O.E. hlafæta "household servant," lit. "loaf-eater." Modern monosyllabic form emerged 14c. The verb meaning "to play the lord, domineer" is from 1377; to lord it is from 1579. Interjection Lordy first attested 1853, Amer.Eng. Lord of the Flies translates Beelzebub (q.v.) and was name of 1954 book by William Golding.

So while mister is derived from master, it appears to never have had the same connotation as master. Sir began to be used as a respectful form of address in civil society, not as a deferential form. Interesting. I don't see any reason why they can't be used in a more egalitarian society. (As they are, currently.)

However, why is sir used to address superiors in the military? Because of its link to sire?

Wikipedia:

It was once used (without the person's name) as a courtesy title among equals, but in common usage it is now usually reserved for one of superior rank or status, such as an educator or commanding officer, or in age (especially by a minor); as a form of address from a merchant to a customer; in formal correspondence (Dear Sir, Right Reverend Sir); or to a stranger (Sir, you've dropped your hat).

The equivalent for a woman when used as a term of address is "madam" or "ma'am".

If by convention here in the U.S. sir is used only for superiors, then what does one say to equals as a form of respectful address? Mister? Children can say that to adults who are strangers to them, but between adults, it sounds awkward, and that makes some sort of introduction necessary. But what if such an introduction is to be avoided? How should a police officer address someone? (I don't think they say anything except "you" when they are questioning someone.) If sir gives the wrong impression that the one saying it is an inferior, then what should replace it?

What did they do in the South? (I suppose an answer could be found in Northern social history as well.)

More reflections in this post and this one.