In sight seeing in Paris, Twain visited some of the faubourgs and speculated that some of the residences belonged to "lorettes". I guessed the innuendo correctly.
From the Internet:
"The middle ground between street prostitute and grand dame of commercial sex, the courtesan, lorette became an umbrella term for the kept women set up discreetly in a private apartment by a businessman, professional, or wealthy student... Always elegantly dressed, the lorette peeps out coyly from a theatre box, engages in double entendre with male admirers at a masked ball, displays herself while enjoying the view from her apartment window... the lorette slid imperceptibly across the boundaries of acceptability and social stigma."

-Nicholas Green, The Spectacle of Nature