I'm wondered about the history of the title "First Lady" as it's given to the wife of a sitting US president. The role itself seems to have some evolution over time, with even non-wives of the President fulfilling it.
The Wikipedia entry has some information, related to James Buchanan's wife and an eulogy by Zachary Taylor for Dolly Madison, but no definitive citations regarding the usage becoming common or accepted. Etymonline has some information about the use of "Lady" as a title (as in Ladies and Gentlemen), but the site dates the common usage of the honorific to around 1890, well after Buchanan's presidency.
The second part is related to the Titles of Nobility Clause in the US Constitution. The title "Lady" is commonly accepted as the feminine of "Lord", is the exception due to the common usage of the honorific?
While the title of first lady was used to describe the president's wife throughout the early history of the nation, it began to be used as an unofficial official title during James Buchanan's Presidency.
James Buchanan wasn't married. There is some evidence that he may have been a homosexual, but I won't go into that. Regardless of his sexuality he wasn't married when he became President and so to fulfill the roles of the "president's wife" (which was usually what people used to refer to the First Lady before Buchanan) Buchanan brought in his Niece, Harriet Lane. Because she wasn't married to Buchanan you couldn't call her the president's wife, and so people referred to her has First Lady. Now the true origins of that term are usually attributed to the Americans using the respectful term "First Lady" on Martha Washington and Dolly Madison specifically, and all previous president's wives broadly. The reason they referred to them in this way comes from the Romans. One of the titles of the Roman Emperors was "Princeps" which can be roughly translated into "First Man" or "First Citizen". The idea around calling the Roman Emperor this was that the Emperor wasn't a king; he was just the first citizen of Rome and commanded respect because of that. So when Zachary Taylor called Dolly Madison the "First Lady" he was calling back on that old tradition.
EDIT: Also let me add some sources as I was on mobile when I posted earlier
The information on the title "Princeps" I got from "Augustus", by Anthony Everett.
The Information on Buchanan's sexuality and his niece I got from both "A Country of Vast Designs" By Robert W. Merry, and the History Channel series "The Presidents of the United States"
I also want to add a bit more information. There is no written record of the Zachary Taylor eulogy about Dolly Madison. Instead the first written record of "First Lady" being used as a title is in reference to Mary Todd Lincoln (Source: "Team of Rivals" by Doris Kearns Goodwin) . This leads me the the working theory that it was used as Harriet Lane's title, in reference to the Zachary Taylor eulogy and/or the Roman stuff, and was cemented by her immediate predecessor, Mary Todd Lincoln. Then from there the dominance of "Lincoln's party" for the next 40-some years meant that every president wanted to call back to his administration, and the name stuck.