US Army slang common during the Vietnam War, although possibly originating much earlier than this. It means a soldier who is a combat virgin who has not been 'blooded' and, therefore, one whose behaviour in combat is unpredictable. The arrival of a new soldier as a replacement in an established unit was always difficult, hence the referral to such an arrival as FNG (Fucking New Guy). Where this soldier had not seen combat the difficulty was compounded and the established troops viewed them with suspicion as an unknown quantity.
This is US Army slang commonly used during the Vietnam War. It was also used during the Korean War and may even date back to World War II. It means a soldier who is a combat virgin whose behaviour in combat is, therefore, unpredictable. When soldiers like this arrived in a unit, as a replacement for someone killed or wounded, they were often viewed with suspicion and were referred to as FNGs, until they had proved themselves.