The nickname of Billie Holiday, given to her by her pal Lester Young. Sometimes people shortened it to "Lady"; sometimes it was "Miss Lady", if you were being a proper gentleman. Young, in turn, was given the nickname "Lady Lester" by Miss Holiday. That nickname didn't gain wide currency for some reason; he became better known as "Prez", as in "The President", another name given by "William" Holiday to Lester.