SWAG is an acronym that stands for "Stuff We All Get", although it is commonly and jokingly explained as "Secretly We Are Gay" or "Sex With A Girl".
People who use SWAG alot use it when talking about something cool or sexy, however using SWAG seriously will just make you look retarded.
(In the distance) Douchebag: Oh man I've got so much swag!
Person 1: Man that guy's such a retard.
Person 2: ikr