Swag was first used for rap. Swag is the abbreviation of swagger, which is hiphop trend. Many people will add a swag after rap, which is roughly a state of self-confidence and style. It may be close to the meaning of another more common English sentence "just do it". It can be understood as the coolest, the most powerful, the sixth most awesome ... and so on, which is an obvious modal particle.
This word is often used by some young people who love hiphop culture. Swag is the most commonly used hip-hop cultural word among young people in China. Some students talk about swag, some take swag as their own signboard, and what's more, there will be several clothes printed with swag in the closet. However, people who are related to pop culture and catch up with fashion all say they are swag. The word is like a flood.
However, this word became popular again on the Internet because of the broadcast of a Korean drama "Kim Zhu Fu, the Lightweight Fairy", because the protagonist in the drama often takes this word as a mantra, and many fans can say "swag" to follow the trend even if they don't know its specific origin and cultural significance, so this word is very popular with fans.