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c.1300, "the Holy Grail," from Old French graal "Holy Grail, cup," earlier "large shallow dish," from Medieval Latin gradalis "a flat dish or shallow vessel," perhaps ultimately from Latin crater "bowl," from Greek krater "bowl, especially for mixing wine with water."
Holy Grail is anglicized from Middle English Sangreal (Saint graal), grafted awkwardly onto the Celtic Arthurian legends 12c. by Church scribes probably in place of some pagan otherworldly object. It was said to be the cup into which Joseph of Arimathea received the last drops of blood of Christ (according to the writers who picked up the thread of Chrétien de Troyes' "Perceval") or the dish from which Christ ate the Last Supper (Robert de Boron), and ultimately was identified as both (e.g. "þe dische wiþ þe blode," 14c.).