Un Violon D'Ingres

Un violon d'Ingres (silent s) is not to be confused with "un violon dingue"  (a crazy violin) - although you could potentially have un violon d'Ingres dingue - a crazy Ingres's violin.  The violin in question here belonged to the French neoclassical portrait painter Jean-Auguste-Dominique Ingres, who lived between 1780 and 1867.

Did Ingres use his violin to paint?  Nope.  Then what's the violin to do with painting? Nothing.  Nothing at all.  Ingres's violin was in fact...his violon d'Ingres!  It was his hobby, what he enjoyed doing after work, when he was done painting.  And that's how the phrase is still used today: un violon d'Ingres is an interest pursued for pleasure and not as a main occupation, over a long period of time - if not your entire life, at least a big chunk of it.

Why such a craze around Ingres and his violon that it turned into an expression?

Origin

Let's look at his life first.  From the ages of thirteen to sixteen, Jean-Auguste Ingres was a student at the Académie Royale de Peinture, Sculpture et Architecture of Toulouse, a city in south-western France.  At the same time, he was second violinist in the Orchestre du Capitole.  He then moved to Paris to attend the Ecole des Beaux Arts and went on to win many prizes and have a brilliant career as an artist.  All the while he would continue to practice his instrument and volunteered to play at every opportunity throughout his life.  He was known for his art, but in his heart, violin came a very close second.

Ingres's passion for his hobby inspired little known novelist Claude Farrère to write L’Homme qui assassina (The Man who killed – 1907.)  The book tells the story of Narcisse Boucher, the French ambassador to Constantinople, who in his youth had been a violin prize winner at the Conservatoire of Paris. Although his musical career seemed all mapped out, he became a businessman, then a politician.  The author concludes that, despite Boucher’s complex personality,

le violon est là, le violon d’Ingres, pour tout envelopper, diplomatie et finance, d’une harmonie imprévue, plus paradoxale que tout le reste.

the violin is there, le violon d’Ingres, to envelop everything, diplomacy and finance, in an unexpected harmony, more paradoxical than all the rest.

Thus was born the figurative use of violon d’Ingres in a French text.

The American avant-garde artist Man Ray, who was an admirer of Ingres, then used this phrase as the title of a famous photograph portraying model Kiki de Montparnasse.  Ray altered what was a classical nude by adding the f-holes of a stringed instrument onto the photographic print.

The title seems to suggest that, while playing the violin was Ingres's hobby, toying with Kiki ..... was Man Ray's past time.

Examples

Collecter tous ces objets de valeur, c'est mon violon d'Ingres.
That's my hobby, collecting anything of value.

Désolée, le billard n'est pas mon violon d'Ingres.
So sorry, pool isn't my game.