The French phrase violon d’Ingres denotes a hobby, i.e. an activity done regularly in one’s leisure time for pleasure—cf. another French term, dada.

This phrase refers to the passion that the French painter Jean Auguste Dominique Ingres (1780-1867) had for playing the violin; from the ages of thirteen to sixteen, when he was a student at the Académie Royale de Peinture, Sculpture et Architecture of Toulouse, a city in south-western France, he was second violinist in the Orchestre du Capitole, and he would continue to play this instrument for the rest of his life.

In Peintres mélomanes: l’apothéose de Mozart et le violon d’Ingres, published in Le Ménestrel: journal du monde musical (Paris) of 6^th January 1901, Raymond Bouyer evoked the fact that Ingres bequeathed his violin, together with the whole of his paintings, to his native town, Montauban, near Toulouse; about “le petit musée” of this town, Bouyer wrote:

Là, dans une vitrine d’honneur, fut déposé, par son expresse volonté, le violon d’Ingres.
     translation:
There, in a showcase of honour, was deposited, following his express wish, Ingres’s violin.

Ingres’s violin was carefully boxed up and kept in reserve before renovation of Ingres Museum in Montauban:

 

The earliest known figurative use of violon d’Ingres in a French text is from L’Homme qui assassina (The Man who assassinated – Paris, 1907), by the French novelist Claude Farrère (Frédéric Charles Bargone – 1876-1957). The author draws the portrait of Narcisse Boucher, the French ambassador to Constantinople, who in his youth had been the violin prize winner at the Conservatoire of Paris; although his musical career seemed all mapped out, he became a businessman then a politician; however, sometimes, during the soirées that he gives, he plays the violin before his guests; 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. Narcisse Boucher, c’est, d’abord, un dilettante…
     translation:
the violin is there, le violon d’Ingres, to envelop everything, diplomacy and finance, in an unexpected harmony, more paradoxical than all the rest. Narcisse Boucher is, first and foremost, a dilettante…

However, curiously, violon d’Ingres had been used figuratively earlier in an English text; the following passage from the Parisian news, published in The Bystander (London) of 24^th January 1906, is about Paul Doumer (1857-1932), Governor-General of French Indochina from 1897 to 1902, President of the French Republic from 1931 until his assassination:

M. Doumer’s “Violon d’Ingres” is his horse-riding. He is prouder of an eighty or ninety kilomètre ride towards Saigon or Hanoi, when he returned as fresh as the morning, leaving his followers thirty or forty miles on the road extenuated, than of all his administrative triumphs. He, who unified the budgets in Indo-China by drawing together Cambodia, Anam, Tonkin, Laos, and Cochin-China; who obtained the railway grant by his eloquence; who made the Colony a financial success, is the proudest of men when he talks of his powers as a horseman.