It’s curious. In Celtic legend and myth there is no holy grail. However, in those references most closely tied to the story, there is a Dysgyl. There are several English translations of the word, including body. That is likely where the connection to the grail containing the holy blood of Christ came from.
However, the context of the word in all the Celtic versions, including the Welsh version of Le Conte du Graal, makes clear the intended interpretation is cauldron. One can go a step further and connect all these instances with a witch or witches. In the Celtic tales, the cauldron is a symbol of fertility and is ofen destroyed or stolen by the hero, such as in the cases of Peredur (Perceval) and Arthur.
Chrétien de Troyes was the first individual to call the object a holy grail, a sangraal. It was not a popular connection at first. Wolfram von Eschenbach, a better knight and an important writer as well, called it a black stone. It did not come into the public consciousness until the Arthur stories were first coalesced into a single and consistent timeline of events a couple hundred years later. One must wonder what the writers of The Vulgate Version of the Arthurian Romance and Sir Thomas Malory would have thought of the Knights of the Round Table searching for a witch’s cauldron!