La belle ferronnière is a portrait of a lady, usually attributed to Leonardo da Vinci, in the Louvre. It is also known as Portrait of an Unknown Woman. The painting’s title, applied as early as the seventeenth century, identifying the sitter as the wife or daughter of an ironmonger (a ferronnier), was said to be discreetly alluding to a reputed mistress of Francis I of France, married to a certain Le Ferron. The tale is a romantic legend of revenge in which the aggrieved husband intentionally infects himself with syphilis, which he passes to the king through infecting his wife.[citation needed]

Leonardo’s Lady with an Ermine, has also been known by this name. This was once believed to be a portrait of Cecilia Gallerani—one of the mistresses of Lodovico ‘il Moro’ Sforza, Duke of Milan.[1] The narrative and the title were applied to Lady with an Ermine when it was in Princess Czartoryski’s collection, and became confused with La Belle Ferronniere by the presence in this image also of a jewel worn on a delicate chain across the forehead, called a ferronnière.
Source: La belle ferronnière – Wikipedia