knickersyoudaoicibaDictYouDict[knickers 词源字典]
knickers: [19] The use of the word knickers for ‘women’s underpants’ dates back to the 1880s: a writer in the magazine Queen in 1882 recommended ‘flannel knickers in preference to flannel petticoat’, and Home Chat in 1895 was advertising ‘serge knickers for girls from twelve to sixteen’. Over the decades, of course, the precise application of the term has changed with the nature of the garment, and today’s legless briefs are a far cry from the knee-length ‘knickers’ of the 1880s.

They got their name because of their similarity to the original knickers, which were knee-length trousers for men (The Times in 1900 reported the ‘Imperial Yeomanry … in their well-made, loosely-fitting khaki tunics and riding knickers’). And knickers itself was short for knickerbockers, a term used for such trousers since the 1850s. This came from Diedrich Knickerbocker, a fictitious Dutch-sounding name invented by the American writer Washington Irving for the ‘author’ of his History of New York 1809.

The reason for the application seems to have been that the original knickerbockers resembled the sort of kneebreeches supposedly worn by Dutchmen.
[knickers etymology, knickers origin, 英语词源]
knickers (n.)youdaoicibaDictYouDict
"short, loose-fitting undergarment," now usually for women but not originally so, 1866, shortening of knickerbockers (1859), said to be so called for their resemblance to the trousers of old-time Dutchmen in Cruikshank's illustrations for Washington Irving's "History of New York" (see knickerbocker).