Egghead is such an odd insult. Is the implication that their head is large due to excess intelligence, and somehow that is a negative? And by pointing it out as a negative, there is the implication that you don’t think you reflect that quality, so you’re inadvertently saying “I don’t like you brainiac, because you’re not a fool like me.”

I guess it’s pretty hard to insult people for traits that have been almost universally considered positive for the majority of human history.