According to etymonline.com, the work “Esquire” is a noun. It came to use “in the late 14C., from Middle French esquier “squire,” literally “shield-bearer” (for a knight), from Old French escuier “shield-bearer (attendant young man in training to be a knight), groom” (Modern French écuyer), from Medieval Latin scutarius “shield-bearer, guardsman” (in classical Latin, “shield-maker”), from scutum “shield” (see escutcheon). For initial e-, see e-. Compare squire (n.). Originally the feudal rank below knight, sense broadened 16c. to a general title of courtesy or respect for the educated and professional class, especially, later, in U.S., regarded as belonging especially to lawyers.
In our own dear title-bearing, democratic land, the title of esquire, officially and by courtesy, has come to include pretty much everybody. Of course everybody in office is an esquire, and all who have been in office enjoy and glory in the title. And what with a standing army of legislators, an elective and ever-changing magistracy, and almost a whole population of militia officers, present and past, all named as esquires in their commissions, the title is nearly universal. [N.Y. Commercial Advertiser newspaper, quoted in Bartlett, 1859]
Meanwhile, Wikiquote tells us: “Esquire (abbreviated Esq.) is a term of British origin (ultimately from Latin scutarius in the sense of shield bearer via Old French “esquier”). In Britain, it is an unofficial title of respect, having no precise significance, which is used to denote a high but indeterminate social status. Esquire is cognate with the word squire, which originally meant an apprentice or assistant to a knight. Relics of this origin can still be found today associated with the word esquire. For example in the Most Venerable Order of the Hospital of Saint John of Jerusalem, “Esquire” is today the most junior grade of membership. In the United States, the suffix Esq. most commonly designates individuals licensed to practice law, and applies to both men and women (in more modern times).”
Historically, in the UK, “esquire” was a title of respect, sometimes referred to as a courtesy title, accorded men of higher social rank, especially those members of the landed gentry who were above the rank of “gentleman,” but below the rank of “knight.” William Blackstone, a renown judge and jurist and author of Commentaries on the Laws of England said of the subject, “The title should be limited to those only who bear an office or trust under the Crown and who are styled ‘esquires’ by the king in their commissions and appointments; and all, I conceive, who are once honoured by the king with the title of ‘esquire’ have a right to that distinction for life.”
A gentleman was considered to be any man of good and courteous conduct. Originally, it was the lowest rant of the landed gentry of England, ranking below both “esquire” and “yeoman.” The rank of gentleman was comprised of the younger sons of the younger sons of peers and the sons of a baronet, a knight, and an esquire, in what is known as perpetual succession. A gentleman was not only courteous and law abiding, but he could display a coat of arms, which was a right he shared with members of the peerage, as well as some of the gentry. These groups equaled the British nobility.
In the 17th century, in Titles of Honour (1614), the jurist John Selden said that the title gentleman likewise speaks of ‘our English use of it’ as convertible with nobilis (nobility by rank or personal quality) [Selden, John (1614). Titles of Honour (1st ed.). London: William Stansby for Iohn Helme] and describes the forms of a man’s elevation to the nobility in European monarchies. In 1827, James Henry Lawrence explained and discussed the concepts, particulars, and functions of social rank in a monarchy, in the book On the Nobility of the British Gentry, or the Political Ranks and Dignities of the British Empire, Compared with those on the Continent. [Lawrence, Sir James Henry (1827) . The Nobility of the British Gentry or the Political Ranks and Dignities of the British Empire Compared with those on the Continent (2nd ed.). London: T.Hookham — Simpkin and Marshall.]
Esquire was not in general use for solicitors. More likely to be used by barristers. It was the form used by all those grandsons of peers without any other titles.
The rules of precedence of the Regency period put “esquire” and “gentlemen” in different categories. Landed men, especially those related to peers, like Fitzwilliam Darcy in Jane Austen’s “Pride and Prejudice” would be an “esquire.” He was the grandson of an earl. All the sons of younger sons of the peerage would be an “esquire” as would be sons of knights and baronets. Their sons would be gentlemen as would those with the king’s commission as an officer and a gentleman. The lines between esquire and gentleman were often hard to distinguish for all except the College of Heralds, and they charged a fee to make the decision.
“Esquire” was a status on the table of precedence.
An “esquire” was also a barrister or a judge who had not been given a peerage or even a knighthood. Younger sons of younger sons of dukes and marquesses or sons of earls, viscounts, and barons might be presented “esquire” after their names. Professors usually used their academic degrees, but would probably be seated with the esquires. The sons of a baronet ranked there.
A knight is a title senior to “esquire” for a barrister, for example. William Garrow was both. He was Sir William Garrow, PC, KC, FRS. (No “esquire.) Once knighted, he would be called Sir William. [On a side note and of interest to me with 40% of my ancestral DNA being from Scotland, Garrow was descended from the Garriochs of Kinstair, a Scottish royal line.]
“Esquire” was not used in speech, but, more so, perhaps, in addressing a formal letter.
If this topic interests you, please consider reading In Britain, who is entitled to the suffix of “Esquire” (“Esq.”)? It is MUCH more detailed than what I have attempted to cover here.