Unlike what a lot of rarts think this is not an acronym, and a rapper definetly didnt invent it.
Swag is in fact just a cosa nostra lingo, meaning stolen clothes, commonly from highjacked trailers with clothing shipments. As these are intended for resell the clothes are usually from expensive brands.
The term was first used in the film Goodfellas (1990), and it's been used twice over the course of the sopranos (1999-2007).
-We got some dior dresses for you, it's swag