Why exactly is a U.S. marine referred to as
a jarhead --- or a leatherneck?

LEATHERNECK:

A nearly universal moniker for a U.S. Marine, the word originally described the wide, stiff piece of leather collar, called a stock, worn as part of the Marine uniform from 1792 until 1872. Its purpose was twofold: it protected the neck and jugular during battle and it also kept the Marines' heads erect during more ceremonial occasions. A testament to the widespread use of this term: the official Marine Corps Association magazine is titled Leatherneck.

JARHEAD:

Originating during World War II, this is another reference to official Marine neck-wear. The Marines' Dress Blues uniform features a tall, stiff collar and sailors, in their seemingly never-ending pursuit to poke fun at the Marines, noted how it the collar made Marines' heads look like they were sticking out of the top of a jar, hence the term Jarhead.

Please don't give me any credit. I googled it.

Hey, you get the credit, Tree; you got me the answer no matter how. Thanks, girlfriend!