What exactly does the term NOS mean when referring to body parts?

New Old Stock

It means it either came from the manufacturers production or service stock - NOT repops !! New Old Stock also means that the parts has never been used on a vehicle.

I have heard that some NOS body panels were actually rejects from the factory (although I can't confirm this...). I suppose this makes sense though because GM can return rejected production parts to the supplier and I'll bet he doesn't just toss 'em in the trash....

I hear of some guys with NOS parts that "test fit" on their car - like fenders or something. This is still NOS as it hasn't seen any miles...