It is a shirt. This is a western shirt/cowboy shirt styled after a type of fashion designed by a man named Nudie Cohn...typically called "Nudie Suits". The originals were worn in the 1950's and 1960's by many big country music stars in the US.
I am a photographer and just got back from almost a week doing an IndyCar shoot in Alabama. One of the restaurants we frequented while down there had a picture of Gram Parsons from the Flying Burrito Brothers (a '60s Country Rock Band) wearing his Nudie Suit so the whole cowboy clothes thing was on my mind. I thought maybe I had a black shirt which would go well with the black Speedy.
Here is Nudie Cohn himself wearing one of his namesake suits.