Hold one picture: a single cup from an old story, and a line of knights riding away from the castle to look for it. They give their whole lives to the search; almost none of them come back with it. Everything the phrase does in modern English comes out of that scene — one thing, wanted by many, always one step further on.
The packet's Wiktionary entry records the name as coming from Middle English Holy Grayle; the modern figurative use is presented here as a borrowing of the legend, not as a further historical step.
來源資料記載這個名字來自中古英語 Holy Grayle，指的就是傳說中那只杯子。今天的比喻用法，在這裡只當成「把故事借過來用」，不是在說歷史上又演變了一次。
Wiktionary, "Holy Grail"
One branch
那只找不到的杯子，和它的比喻
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the cup in the legends · noun
In old European legend, the cup that Christ is believed to have used before he died. In the stories of King Arthur, knights spend their lives searching for it, and in later versions it is also the cup that caught his blood.
聖杯