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.