We've all been there. Sitting in Boot Camp or OCS, I assume, and listening to the stories of the "Old Corps". Do you remember the tales of sword fights from one ship to another? Marines swinging devil may care on overhead lines (what do they call them anyway?) Swords slashing one anothers limbs and necks. The "Leatherneck" was created to protect our necks from the swords so we could live to fight another day.
Or... ummm...maybe not.
25 March 1804 the first official uniform order was issued by the Marine
Corps and approved by the Secretary of the Navy directing Marine Officers
to wear "black leather stock (collars) when on duty." The wearing of these
heavy leather collars is believed to be the foundation of the nickname
"Leatherneck" for the U.S. Marines. While the leather stock, worn by Marines
from 1775 to 1881, was intended to resemble the high stocks and collars
of the early years, it was retained by the Marine Corps to make the Marine
keep his head erect while in uniform.
Source: United States Marine
Guidebook of Essential Subjects
The term "Leatherneck" as applied to Marines is widely used but few people associate it with the uniform. The fact that United States Marines wore a black leather stock, or collar, from 1798 to 1880 may have given rise to the name. According to tradition, the stock was originally worn to protect the jugular vein from the slash of a saber or cutlass. However, official records fail to bear this out.
Source: Guidebook For Marines