An
overly intellectual person; someone who thinks too much.
Example:
1)
Charles can't order lunch without using an algebraic formula -- what an
egghead!
2) Those eggheads in the media are always giving
advice, but they don't know anything about real life.
Etymology:
Presumably, someone who thinks a lot must have a big brain, and their head
must be large enough to hold their brain. A large head might be shaped
something like a giant egg.
