Not so long ago, I was involved in some back and forth over the use of the title Doctor by people having been given an honorary degree. Along the way, I came across some other information about titles.
The original title, Esquire, comes from the Middle Ages — an esquire was the rank above a gentleman, and below a knight. The word derives from the Latin scutum (shield), and Middle English esquier (shield bearer). It was also generally applied to upstanding members of society: judges, sheriffs, lawyers, professors.
The title was eventually adopted almost exclusively by the British legal profession. There were still plenty of non-lawyer esquires (the title was often passed down to eldest sons), but it was very popular among barristers. The title was often shortened to "Squire."
Today in the United States, the title is mostly used as a sign of courtesy toward members of the legal profession. It also pops up in the stationery of lawyers who think of themselves highly. It’s a formal address that has no specific meaning.
However, if you want to get, um, legal about it, the use of the title "esquire" could be downright treasonous. The U.S. Constitution forbids aristocratic titles. The Articles of Confederation state that: "nor shall the United States in Congress assembled, or any of them, grant any title of nobility."