In our world Jack is a commmon name, it is a variation of "John" which is taken from "Ioannes" which was taken from "Yochanan" which means "God is gracious".



Don't ask me how it happened, this is just what the name etymology site tells me.



Also, centuries ago here on Earth, "Jack" became a slang for "man" and thus was used as a stock fairytale character name. Particularly one about a man known as "Stingy Jack" became the legend behind the carved squashes, an old legend about a man who was a thief and scoundrel who tricked the devil into not getting his soul, but when he died he couldn't go to either ends of the afterlife and was doomed to wander the Earth, with only an ember from Hell to light his way around the dark abyss between worlds. He was then known as "Jack of the Lantern", later shortened to "Jack-o-lantern".



((I did look this shit up 0_0))