c. 1300, gral, "the Holy Grail," from Old French graal, greal "Holy Grail; cup," earlier "large shallow dish, basin," from Medieval Latin gradalis, also gradale, grasale, "a flat dish or shallow vessel." The original form is uncertain; the word is perhaps ultimately from Latin crater "bowl," which is from Greek krater "bowl, especially for mixing wine with water" (see crater (n.)).
Holy Grail is Englished from Middle English seint gral (c. 1300), also sangreal, sank-real (c. 1400), which seems to show deformation as if from sang real "royal blood" (that is, the blood of Christ) The object had been inserted into the Celtic Arthurian legends by 12c., perhaps 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 ("þe dische wiþ þe blode," "Joseph of Aramathie," c. 1350?).