I came across leman in a book by Anya Seton set in Elizabethan England - it was apparently in use then. I had never heard the word before so went searching in a dictionary; I don't know when it ceased being common usage. The context of use in that book led me to assume it referred to an unmarried woman who was involved in a sexual relationship with a man but possibly not as formal an arrangement as being a mistress would have been. Mind you I really like the term "floozie" for someone's light-of-love.
Cheers
JennieG