The slang term “egghead” has been used since the beginning of the 20th century. It refers to a man who is either an intellectual or who has intellectual pretensions. Such an individual is thought to have a deeply receding hairline, i.e., a “high brow.” This tall forehead does indeed give the face a sort of egg shape. The term was not originally applied to Adlai Stevenson 1922 but to those who would vote for him.
Editor’s note: While the Oxford English Dictionary traces “egghead” back to 1907, its listing also shows that use of the term picked up significantly in 1952, with citations specifically referring to supporters of Stevenson’s first presidential campaign.