| According to Florence King's Reflections in a Jaundiced Eye, the term "First Lady" for the wife of a United States President is actually borrowed from Engish royal/noble titles. On occasions when a King of England was unmarried (no Queen Consort), his mother was not living (no Queen Mother/Dowager), and there is no Princess Royal (traditional title of the king's oldest sister), then the highest-ranking woman in the kingdom would be the wife of the oldest of the Royal Dukes (the king's brothers and in some circumstances uncles). She would go first in processions and get other queenly privileges, so she was designated "First Lady of England," though she would more commonly use her original title of Duchess. For example, John of Gaunt's wife Katherine, Duchess of Lancaster, was the First Lady of England after John's nephew Richard II's first wife died until Richard's second marriage.
I have found references to the term "First Lady of England," notably at
http://www.swynford.force9.co.uk/johngaunt.htm, but I have not found any confirmation that the American title was chosen because of the English usage. | 
