A leatherneck is a marine, the name coming from the high, leather collars once worn by Royal and U.S. Marines. The term is originally British, applied by sailors to marines. It dates to at least 1889, when it appears in Barrère & Leland’s Dictionary of Slang:

Leather-necks (naval), a term for soldiers; from their leather stock, which, to a sailor, with his neck free from any hindrance, must appear such an uncomfortable appliance.

It makes it American appearance in the December 1907 issue of Army & Navy Life:

“Yah, yah, twelve eighty and a horse blanket; yah, yah, leather neck!”
“Sergeant, who the devil is that?”
“An ex-flat-foot who is driving a truck, sir; he passes every morning and devils the man on watch at the door.”

It is likely that the reference to U.S. Marines is borrowed from the British instead of coming directly from the marine uniform. U.S. Marines abandoned their leather neck stocks in 1875, long before the term leatherneck came into use.

(Source: Historical Dictionary of American Slang)

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