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an informal term for a member of the United States Army or Marine Corps, especially used to refer to members of the American Expeditionary Forces in World War I, but initially used in the Mexican-American War of 1846-48

found this but dont know if it will help I hope it does!
it is a nickname used for U.S. infantrymen that were deployed out to Europe. the origins of the nickname are not completely know but it is suggested to go back to the Mexican-American war.