feu gregeois is French for "Greek fire" - it was lobbed at enemy ships (which were of course made of wood) during naval battles to set fire to them. I am not sure when it was invented. I believe it was used by the Byzantines in the Middle Ages.
"Grégeois" is a term that can refer to the Greek language or a person from Greece. It is also used to describe items or characteristics associated with Greek culture.