Leatherneck
The term leatherneck stuck with the Marines because the Marines stuck with the leather stocks the longest out of the different branches of US services. To paraphase an Army officer of the 1870s I believe he said something like " the Marines looked like geese constantly looking upwards for rain."
I did not do the research on this but a coleague of mine did in Grad school. It seems for most terms to stick in the English language they need to come from two different perspectives and have a meaning that is somewhat linked.
First the use of a leather stock predates US History. Many European armies hd been using them for the purpose to keep a soldiers head straight and erect. You didn't want a soldier looking around too much at the destruction all around him, especially when he is in tightly packed lineal formations. so that is explaination #1
(Tests have been done on some reproduced models of the stocks and some swords do penetrate them and also any direct blow to the neck is going to put you down, maybe still alive, but probably not for long.)
There is also evidence for a non military use of the term that seemed to arise in the 1700s. The term leatherneck seems to be cut from the same cloth as the term "redneck." Redneck as many may know is a term given to a rural person , usually a farmer because all the time in the field would burn the exposed neck and thus turn it red. Leatherneck on the other hand seems to be a description of a person who does hard manual labor, much like a "roughneck". The hard labor producing sweat and grime on top of the red, burned neck would, over years, create a "leathery" appearence. So like red neck leatherneck becomes a sort of put down for poor hard labor workers. So that is explaination #2. ( as an example of this see my father's neck. A CWO-4 Marine and Marine Reservist for 42 years and a construction worker when not on duty, his neck and those of his brothers, all likewise Marines and working men afterwards, look like leather!!!)
Now when we consider those who would have been most likely to have joined or be conscripted into the military in the past would predominantly have been poor laborers and farmers with these "leathery necks" would now be wearing the leather stocks this could be the genesis of the general term.
As a Marine Corps Term it seems that a recruiting drive in the 1920's led to the creation of a few well intentioned legends ie.. The term Leatherneck for Marines. Tun's Tavern as the Birthplace of the Marine Corps, the idea that the quatrafoil on top of the Dress Blues cover representing a piece of rope Marine Officers in the sailing navy put on their heads to make them stand out to the Marines in the tops. There are others I can't recall.
But alas, Leathernecks are Marines and Marines are Leathernecks and it is a proud name and tradition that will be forever intertwined.
Semper Fi and
Fortitudine
Colin Murphy
USS Constitution 1812 Marine Guard
and
United States Marine Corps Historical Company