A slang term used to desribe a person who has sexual/intimate relations with their cousins and possibly other close family members. Today, it is commonly used to describe people who have no social skills, are unintelligent or just look stupid. Its origins come from a large, wealthy private school that was large enough to form its own upper class suburb in the north shore of Sydney, Australia.
"Stop spooning your cousin you jed!"
Person one: "Hey Andy I don't think i'll go out on saturday night 'cause I have to watch the third season of Gossip Girl."
Person Two: "Seriously, you are such a jed."