Well I knew that... The land of Canaan was named after its people's patriarch, who was not the same person as Cain, although his name may have been derived from Cain in his honour. It's clear in the Old Testament history that the Canaanites were enemies of the Jacobites. The Canaanites subverted the Jacobites, most notably with the seduction of King David by Bathsheba, who was a Canaanite, and the mother of King Solomon. Thus they adopted the name 'Jews' from the tribe of Judah, son of Jacob. So we know from the etymology of the word 'cannibal' and the history of the Jews/Canaanites that the Jews were cannibals and, assuming they've preserved their cultural practices secretly, they still are to this day. But that doesn't show that Cain was a cannibal. I find the idea totally plausible that he was, but there's no actual evidence or record of it that I'm aware of.