Commonly used to describe someone who is attractive and has an aesthetically pleasinglower back/behind.
Also a lyric in Mic Drop 'I got hella trophies and is hella thick'
In BTS song Mic Drop ft. Desiigner, the English part says "Did you see my bag? Do you see my bag? It's hella trophies and it's hella thick". They are referring that since they work hard their bag contain lots of trophies and it's heavy (thick). Since the original version is in Korean, when they sang that part in English it was funny so many people made it into a inside joke.