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. [plus fours etymology, plus fours origin, 英语词源]
"eighty, four times twenty," mid-13c., "formerly current as an ordinary numeral" [OED], from four + score (n.). Archaic by the time Lincoln used it at Gettysburg in 1863. Related: Fourscorth "eightieth."