tntstanifer - As a person interested in USMC history, I was very interested in the origins of terms. The history of the term Jarhead is as follows:
There are 2 theories as far as how the term came to be.
1.) Mules - they were called jugheads which also came to mean "fool", the term then was transferred to young Marines who did the hard work and eventually became corrupted and became Jarhead. This term was then used by the other services when talking about Marines. This was to infer that hard work should be given to the Marines to do.
2.) Appearance - The dress uniform of the Marines has a high round collar which some people said look liked a mason jar lid. Another version of this it the popular old style haircut of the Marines. Another version suggest it came from a type of hat that was worn early in the last century.
One fact is that in the 1920s and 1930s Sailors called Marines this term in a derogatory fashion. By the Marines of the day it was considered an insult like someone using a racial term would be.
Other terms that started as derogatory slurs are Grunt and Gyrene.
Most of these terms have lost the derogatory connotation over time, especially when used by one Marine to another, just as some racial slurs are not considered derogatory by some when used by one member of that race to another.
Trust me though, if a non Marine calls a Marine a Jarhead in the wrong way (especially a sailor), trouble can be right around the corner.
Terms that were not considered insults are Leatherneck and Devil Dog.
Leatherneck refers to a high leather collar that part of the uniform.
Devil Dog (teufelhunden) was a term Germans used to describe the fighting prowess of Marine.
Hope that clears it up.