"twilled cotton cloth," mid-15c., Geayne, short for Gene fustian, from Middle French jean fustian "fustian (a type of twilled cotton cloth) of Genoa," the Italian city, from Old French Jannes "Genoa," from Latin Genua (see Genoa). Compare obsolete jane, name of a small silver coin of Genoa that circulated in England 15c. The plural form jeans became standard by mid-19c. In the sense "trousers made of jeans" it is attested by 1908; noted as characteristic of teenagers from 1959. Not originally blue.
After sheep could be protected from the wolves, the people fared better in the matter of clothing. Flannel and linsey were woven for the wear of women and children, while jeans was woven for the men. For want of other dye-stuffs, the wool for the jeans was almost invariably colored with the bark or young shoots of the walnut; hence the inevitable "butternut" worn so extensively in the West for many years. ["History of Menard and Mason Counties, Illinois," 1879]