A vest which is sometimes seen being worn on the gym by the really built users. Importantly, it is only allowed to be worn by somebody especially built - though a fat person is highly unlikely to be wearing one anyway.
The main feature of this vest is that it is very narrow at the front, which means that nipples can easily be seen protruding from the sides - hence its name.
The purpose behind wearing a nipple vest is to emphasize the size of the pectoral muscles. It can also be seen as an intimidating device for the beginners, as if to say "It will take you a long time till you can wear one of these, son"
-"Dude, check out that guy, he is wearing a nipple vest!"
-"Yeah, he must be huge!"