Where did Jarhead come from?

Answer 1

It was meant to be a derogatory term derived during WWII by the Navy. It refers to the fact that, in the Dress Uniform, Marines look like a Mason Jar, and the shaved head of a Marine resembles a Mason Jar because it is "smooth on the outside and empty on the inside."

Answer 2

It is believed to have been granted them by the US Navy who implied that their scalps were smooth and shiny like a Mason Jar and that the jar was quite empty. It was intended as a derogatory slur, but became an honored part of Marine Corps Tradition. Marines do not take it as a put-down, and rather like the term. For Marines few things are of greater importance then the location of the bayonet lug on the newfangled rifle and which direction is the enemy.