Foy is actually a term for the smell that accumulates between a mans balls and his arsehole, as in Gringo's arse and foy smell. It is an unusual musty smell that is surprisingly hard to rid despite a decent personal hygiene regime. Often associated with physical activity and sport.
FOY stands for: the Fountain Of Youth. A magical substance that allows one to stay young for an extended period of time and never age. They must drink or bathe in it for it to work. This legend has been around for millenniums and is still around today.
Evil old witch: I must have the FOY ! it is the only way i will be beautiful again!!
Servant: Yes master. I will search every inch of the world to find it.
Evil old witch: Cackles wickedly