Google comes up with the following explanation by Flaubert:
"I want to write the moral history of the men of my generation-- or,
more accurately, the history of their feelings. It's a book about
love, about passion; but passion such as can exist nowadays-- that is to say, inactive."
The phrase is one that anyone might make up, in any of various senses. But the title appears to be very frequently alluded to,
sometimes ironically & sometimes irrelevantly. In _The Age of Longing_, by Arthur Koestler, it is the title of a chapter detailing the evolution of a revolutionist's son into an apparatchik.
