FNGs

The term "Fucking New Guy" (FNG) is a derogatory term, made popular within combatants, military chaplains, and combat medics of the U.S. Army and the U.S. Marine Corps deployed to South East Asia during the Vietnam War, usually to refer to newcomers.

Usually, but not always, the term referred to recruits fresh from the United States who joined pre-existing units in Vietnam. Every unit had a FNG, and the term was used across all unit types, from front line combat through to support and medical units. The term was not gender specific; female personnel could be FNGs as well.

The FNG phenomenon grew out of the U.S. Armed Forces's individual rotation policy during the Vietnam War, under which individual troops were rotated in and out in twelve-month tours with already deployed units in Vietnam. In other modern American wars before and since, military units have been maintained and have deployed as a whole. During this period, because of the Cold War, the United States faced the need of maintaining a large presence of troops not only in Southeast Asia, but in South Korea and Western Europe as well. The Johnson administration lacked the political capital and will that would have been required to call up the National Guard and Reserves or to convince Congress to extend the tours of duty of draftees beyond twenty-four months. Lacking sufficient ground combat units to sustain a unit-based rotation strategy, the individual rotation policy was adopted.

...
Wikipedia