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knickers [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]
plus fours: plus fours: [20] The term plus fours was introduced around 1920. It is an allusion to the fact that such trousers were made four inches longer in the leg than the standard knickerbockers or shorts of the time, which came to just above the knee.
knickers (n.): "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).
plus (n.): 1570s, the oral rendering of the arithmetical sign +, from Latin plus "more, in greater number, more often" (comparative of multus "much"), altered (by influence of minus) from *pleos, from PIE *pele- (1) "to fill" (see poly-).

As a preposition, between two numbers to indicate addition, from 1660s. [Barnhart writes that this sense "did not exist in Latin and probably originated in commercial language of the Middle Ages."] Placed after a whole number to indicate "and a little more," it is attested from 1902. As a conjunction, "and," it is American English colloquial, attested from 1968. As a noun meaning "an advantage" from 1791. Plus fours (1921) were four inches longer in the leg than standard knickerbockers, to produce an overhang, originally a style associated with golfers. The plus sign itself has been well-known since at least late 15c. and is perhaps an abbreviation of Latin et (see et cetera).