Actually it is speculated that the the origin ist "Sang(re) Real" ( royal blood ) which metamorphosed into San Graal. Looking for it meant to search for the ( possible) lost blood line of Jesus, I think.

“According to the Catholic Encyclopedia this is a false etymology.” — https://en.wikipedia.org/wiki/Holy_Grail#Etymology

It’s been around for a while but was most recently popularised by Dan Brown in The Da Vinci Code.

Ah interesting, thanks! I'll be more explicit about "in English" next time.