According to Lewis Spence (in the Encyclopedia of Occultism, 1920), Jean was a French magician, votary of Apollonius of Tyana. He traveled from town to town, wearing an iron collar and making his living by performing deeds of charlatanry. At Lyons he attained some measure of fame by his miraculous cures and met with the sovereign, to whom he presented a magnificent enchanted sword. In battle this weapon became surrounded by ninescore drawn knives. Jean also gave this prince a shield containing a magic mirror that divulged the greatest secrets. The arms vanished or were stolen.
(Unfortunately, Spence did not state the period or the ruler involved, but this is probably a medieval legend.)
jean / jēn/ • n. heavy twilled cotton cloth, esp. denim: [as adj.] a jean jacket. ∎ (in commercial use) a pair of jeans: a button-fly jean.