Another source says the slang term was used by sailors as early as World War II to refer to members of the Marine Corps, they drew the term from the resemblance of the Marine dress blues uniform, with its high collar, to a Mason jar which at the time was made from blue glass.
Our friend Bob found this on the net as he was looking around..."A term used to describe a US Marine. Although some believe the term is of recent origin (), it has been around for a long time (at least since ). 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.
Leatherneck - This nickname goes back to the leather stock or neckpiece, which was part of the
Marine Corps uniformfrom 1775 to 1875. The leather collar was designed to protect the jugular vein from saber slashes. It also insured that Marines kept their heads erect and maintained military bearing. Although no longer used, it is commemorated by the standing collar on the dress blue and dress white uniform.
Devil Dogs - In the Belleau Wood fighting in 1918, the Germans received a thorough indoctrination into the fighting ability of Marines. Fighting through supposedly impenetrable woods and capturing supposedly untakeable terrain, the men of the 4th Marine Brigade struck terror in the hearts of the Germans, who referred to Marines as the "Teufelhunden", meaning "fierce fighting dogs of legendary origin" or as popularly translated, "Devil Dogs."