PARIS — For over 150 years, the famous painting’s origin was as mysterious as its subject — a meticulous close-up of a woman’s genitals — was considered unspeakable. No head, no arms, one breast: only a torso, finely rendered. Who posed for this notorious nonportrait by the celebrated troublemaker of 19th-century French realist painting, Gustave Courbet?

The painting itself, “The Origin of the World,” was hidden from public view, whispered about but unseen, existing in the back rooms of private collections, or covered over by other paintings.

The work, in the half-hidden possession of the psychoanalyst Jacques Lacan for many years, is now at the Musée d’Orsay in Paris, where it has been on public display since 1995. The American art historian Michael Fried called it, “very likely the most brilliant rendering of flesh in all Courbet’s art.” The feminist art historian Linda Nochlin called the work “pornography” but also “a little masterpiece of overt sexuality.”

And now finally the mystery of its sitter seems to have been solved, thanks to a chance discovery by a mild-mannered French historian toiling in the archives: the torso belonged, with near certainty, to Constance Quéniaux, dancer at the Paris Opera, courtesan, mistress of rich men, companion of a celebrated composer, and — improbably at the end — a well-to-do older lady living on one of the most chic streets in Paris, the Rue Royale.