Common term for US Marines
A Navy friend of mine once tried to convince me that this came about due to the unfamiliarity of Marines with ships' hatches. Supposedly, given that the average Marine is taller than the average seaman, and unaccustomed to ducking to get through ship's hatches, they would repeatedly "jar" their heads by running them into the doorjam.
More believable is that this came about due to the haircut standards in the Marine Corps. Regulations state all male haircuts will be faded from 0 to not more than 3 inches. In practice, I have rarely seen a Marine's hair longer than 1 inch on top, leading to the resemblance to a jar.
See also Leatherneck