The Greek mythological story of the origins of walnuts is pretty racey, but I'll risk the censors to tell what I have found out. Forgive me, all you high school english teachers, if I depend too much on wikipedia, but that's my source!Â
"Apollo, in reward for the king & queen Dion and Amphithea receiving him with great reverence and hospitality, bestowed a gift of prophecy upon their daughters, but imposed a restriction that they should not betray gods nor search after forbidden lore.
"Later, Dionysus, the brother of Apollo, also paid a visit to Dion's house and was received by the mortals with equal hospitality (Dionysus, in Greek lore is know as Bachus in the Roman version. Originally the fertility god, in the Roman telling he was known as the god of wine and pleasure); during his stay, he fell in love with Dion & Amphithea's daughter Carya and lay with her secretly. He then left but, missing Carya, soon returned under pretext of consecrating a temple which Dion had built for him. But Lyco and Orphe, suspecting a love affair between Dionysus and their sister, guarded Carya to prevent her from having intercourse with the god. By doing so they committed a violation of the restrictions imposed by Apollo, so Dionysus, after several warnings and threats, drove the two sisters mad, in which state they ran off to Mount Taygetus, where they were transformed into rocks, and Carya was changed by Dionysus into a walnut tree. From these circumstances later arose the local cult of Artemis Caryatis."