Douglas Harper's Etymology Dictionary
1855, also demimonde, from French demi-monde "so-so society," literally "half-world," from demi- "half" + monde, from Latin mundus "world" (see mundane).\n
\nPopularized by use as title of a comedy by Alexandre Dumas fils (1824-1895). Dumas' Demi-Monde "is the link between good and bad society ... the world of compromised women, a social limbo, the inmates of which ... are perpetually struggling to emerge into the paradise of honest and respectable ladies" ["Fraser's Magazine," 1855]. Not properly used of courtesans. Compare 18th-century English demi-rep (1749, the second element short for reputation), defined as "a woman that intrigues with every man she likes, under the name and appearance of virtue ... in short, whom every body knows to be what no body calls her" [Fielding].
Usage examples of "demi-monde".
At this moment, his principal favorite is Teresa Cabarrus, a woman of society, or one of the demi-monde, whom he took out of prison.