Many people don't know this, but S.W.A.G. is actually an acronym that was created by gay men back in the 60s when homosexuality was widely frowned upon. The acronym stands for "Secretly We Are Gay". Many of these secretly gay men were clothing and interior designers, or in other fields related to fashion. When they would receive overstock or samples of merchandise, they would refer to them as S.W.A.G. in order to let the other person know that they were interested in meeting for a sexual encounter. From there, it spread to refer to the "sugar daddy" relationships that some of these gay men would enter into with the wealthier, more successful members of their group. They would refer to the generous gifts that they would receive from their sugar daddies as S.W.A.G.
The term has recently been adopted into mainstream commercial "hip hop" where there are large numbers of closeted gay men and women who are performers. If you doubt my explanation, just ask yourself whether it's just coincidence that rappers are so obsessed with fashion designers and jewelry, just like the stereotypical gay man.