Another theory on the origin of "jarhead" says that the Marines' uniforms played a part. The uniform has been a source of mockery throughout history, according to Mental Floss. Because it was originally made primarily of leather, Marines were dubbed "leathernecks" by their brothers-in-arms.
The stiff neck, while no longer made of leather, might have contributed to the "jarhead" moniker. According to the National Museum of the Marine Corps, the name came in part from the way Marines look in their dress uniforms, because their heads appear to be poking out of a jar.
The resemblance to a jar wasn't just physical, either. Another theory documented by Mental Floss claims the term "jarhead" was chosen because Marines are trained to be tough on the outside, and keep any thoughts or emotions bottled up inside, like a jar full of jam.
