Old English walhnutu "nut of the walnut tree," literally "foreign nut," from wealh "foreign" (see Welsh) + hnutu (see nut). Compare Old Norse valhnot, Middle Low German walnut, Middle Dutch walnote, Dutch walnoot, German Walnuss. So called because it was introduced from Gaul and Italy, distinguishing it from the native hazel nut. Compare the Late Latin name for it, nux Gallica, literally "Gaulish nut." Applied to the tree itself from 1600 (earlier walnut tree, c. 1400).
As a noun, "the Britons," also "the Welsh language," both from Old English. The word survives in Wales, Cornwall, Walloon, walnut, and in surnames Walsh and Wallace. Borrowed in Old Church Slavonic as vlachu, and applied to the Rumanians, hence Wallachia. Among the English, Welsh was used disparagingly of inferior or substitute things (such as Welsh cricket "louse" (1590s); Welsh comb "thumb and four fingers" (1796), and compare welch (v.)). Welsh rabbit is from 1725, also perverted by folk-etymology as Welsh rarebit (1785).
Originally a fish sauce made from various plant juices, the word came to be used in English for a wide variety of spiced gravies and sauces; "Apicius Redivivus; or, the Cook's Oracle," by William Kitchiner, London, 1817, devotes 7 pages to recipes for different types of catsup (his book has 1 spelling of ketchup, 72 of catsup), including walnut, mushroom, oyster, cockle and mussel, tomata, white (vinegar and anchovies figure in it), cucumber, and pudding catsup. Chambers's Encyclopaedia (1870) lists mushroom, walnut, and tomato ketchup as "the three most esteemed kinds." Tomato ketchup emerged c. 1800 in U.S. and predominated from early 20c.
The dye made from the tree's inner bark was yellowish-brown, and the word was used (from 1861) to describe the Southern army troops in the American Civil War, but the exact reason is debatable. Many Southern uniforms seem to have been this color; perhaps butternut dye was extensively used in homemade uniforms (but the tree's natural range is mostly in the northeastern U.S.); perhaps some of the regulation gray uniforms faded or soiled to this color; perhaps it was because butternut was a nickname for Southerners in the Midwestern states.
Sense of "mere exterior" is from 1650s; that of "hollow framework" is from 1791. Meaning "structure for a band or orchestra" is attested from 1938. Military use (1640s) was first of hand grenades, in reference to the metal case in which the gunpowder and shot were mixed; the notion is of a "hollow object" filled with explosives. Hence shell shock, first recorded 1915. Shell game "a swindle" is from 1890, from a version of three-card monte played with a pea and walnut shells.
"twilled cotton cloth," mid-15c., Geayne, short for Gene fustian, from 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]