A goon is something particular to CF, but it comes essentially from (I think the original source anyway) what you'd call henchmen.
You know, Al Capone goes up against Eliot Ness in a war of words. As Capone says something witty like "now put an egg in your shoe and beat it Ness", the henchman goes "yeah, dat's right! you walk with eggs in your shoes and dey make your socks all sticky. Nyuck nyuck, nyuck"
On CF, it's an endearing term for people who are slight "social misfits". Not particularly thick or stupid, but just who don't quite know how to interact with others, or often miss the point/joke.
There's a bit of goon in all of us