It stands for New Old Stock.
New old stock (NOS), or old stock for short, refers to aged stock of merchandise that was never sold to a customer and is still new in original packaging. Such merchandise may not be manufactured anymore, and the new old stock may represent the only current source of a particular item.[1] There is no consensus on how old a product must be to be NOS, and some people reserve an NOS label only for products that are actually discontinued.
