furlongyoudaoicibaDictYouDict[furlong 词源字典]
furlong: [OE] Furlong ‘eighth of a mile’, which has now virtually died out except in horse-racing terminology, is part of a vocabulary of lengthmeasuring bequeathed to us by the agricultural practices of our ancestors. It originated as an Old English compound formed from furh ‘furrow’ and lang ‘long’ – that is, the length of a furrow ploughed across a standard-sized square field of ten acres.

Since the term acre varied somewhat in its application at different times and places, the length of a furlong could not be computed with great precision from it, but in practice from about the 9th century the furlong was pegged to the stadium, a measure equal to one eighth of a Roman mile.
=> furrow, long[furlong etymology, furlong origin, 英语词源]
furlong (n.)youdaoicibaDictYouDict
measure of distance of roughly 660 feet, from Old English furlang, originally the length of a furrow in a common field of 10 acres, from furh "furrow" (see furrow (n.)) + lang "long" (see long (adj.)). The "acre" of the common field being variously measured, the furlong varied but eventually was fixed by custom at 40 rods. Used from 9c. to translate Latin stadium (625 feet), one-eighth of a Roman mile, and so the English word came to be used for "one-eighth of an English mile," though this led to a different measure for the English mile than the Roman one. Furlong being so important in land deed records (where mile hardly figures) it was thought best to redefine the mile rather than the furlong, which was done under Elizabeth I.