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yard [yard 词源字典]: yard: Yard ‘enclosed area’ [OE] and yard ‘three feet’ [OE] are distinct words, both of ancient ancestry. The former probably goes back ultimately to Indo-European *ghorto-, which also produced Latin cohors ‘court’ (source of English cohort and court) and hortus ‘garden’ (source of English horticulture) and Russian gorod ‘town’ (as in Leningrad).

Its prehistoric Germanic descendant was *gard-, which, as well as providing English with yard, has produced garden, garth [14] (via Old Norse), and the second syllable of orchard. Yard ‘three feet’ originally meant ‘stick, rod’ (a sense preserved nautically, as in yardarm [16]). It goes back ultimately to prehistoric Germanic *gazdaz ‘pointed stick’ (source of the gad of gadfly [16], etymologically the fly with the ‘sting’).

From this was derived West Germanic *gazdjō, which evolved into German gerte ‘sapling, riding cane’, Dutch gard ‘twig, rod’, and English yard. The Anglo-Saxons used the term as a unit of measurement of land, equal to about five metres (what later became known as a rod, pole, or perch), but its modern use for ‘three feet’ did not emerge until the 14th century.
=> cohort, court, garden, garth, horticulture, orchard; gadfly[yard etymology, yard origin, 英语词源]
yard-arm (n.): also yardarm, 1550s, from yard (n.2) in the nautical sense (attested from Old English) + arm (n.1). In 19c. British naval custom, it was permissible to begin drinking when the sun was over the yard-arm.