It's just one of those playground things you'd call your friends, you couldn't call them a dick or any other swear/curse words cos somebody would tell a teacher, so you call them an egg, a stupid egg, a dumb egg. We didn't have anything against eggs per say, it was just a generic put down. Now it's like a quaint 80's-90's reference, people of my age (approaching 40) in New Zealand still use it, with a half smile, everyone knows what you mean by it & nobody takes offense, "stop being an egg" or "good one, egg" is gentler way of saying dumbass.Why an egg? I think you're all being muppets! Except Iljo who is definitely an egg.
Excellent example of the use of the term egg at 1:25