From the French demi-
, meaning partial
, and monde
, meaning world
. A partial-worlder.
Sometimes hyphenated: demi-mondaine.
The term was coined by early 19th century French author Alexandre Dumas, and came to the English language due to the popularity of his works, including The Three Musketeers and The Count of Monte Cristo.
A demimondaine is a woman who belongs to the prostitute class called the demimonde. These women were generally supported by wealthy lovers, serving as escorts, mistresses and sexual playthings.
They were particularly popular in Victorian/Edwardian England, a time when it was common for a man to lose his virginity to a prostitute. The demimondaines ensured that a man of means would never have to associate with a common streetwalker, hussy or dollymop.
Demimondaines lived by an unusual code. They were certainly not women of polite society, but they accompanied the men of polite society. Thus they had to carry themselves as if they possessed honor, but survive as if they didn't. Their ability to live in two worlds was so important that it gave them their name.
Looking back from the early 21st century toward the innocence of the Victorian era, the disillusioned demimondaines are perhaps the easiest people to identify with. A century of atrocity later, the Victorian spirit is probably lost forever. Looking back with the assistance of period literature and historical texts, we cannot share the same blind enthusiasm for the human race that was possible in that day and age. Like a demimondaine, however, we try to suspend our knowledge of harsh realities and enjoy the glitz around us.
As an aside, steampunk fiction also often reflects contemporary pessimism with regard to the Victorian zeitgeist, and more than a few stories from that genre feature demimondaine characters. According to the OED, demimondaines had achieved their place in the public mind as "adventuresses" as early as 1859.
The OED, "demi-monde"
Encyclopaedia Britannica, "Alexandre Dumas"