violon d'Ingres n.

"an occasional pastime, an activity other than that for which one is well-known, or at which one excells," 1963, from French, literally "Ingres' violin," from the story that the great painter preferred to play his violin (badly) for visitors instead of showing them his pictures.

Une légende, assez suspecte, prétend que le peintre Ingres état plus fier de son jeu sur le violon, jeu qui était fort ordinaire, que de sa peinture, qui l'avait rendu illustre. [Larousse du XXe Siecle, 1931]