A term used to describe a US Marine. Although some believe the term is of recent origin (Gulf War), it has been around for a long time (at least since World War II). It has nothing to do with haircuts, hats or headshape. It refers to the Marines propensity to follow orders, regardless of consequences or personal safety. Because of their single-minded willingness to put their duty before themselves, Marines are said to have jarheads...hard on the outside and empty on the inside. It is a good thing there are such men.
Running up a hill to take a machine gun nest is not something that most people would do, but a jarhead will do it everytime he is ordered.