Illiterate slang for "so much fun". Use this and people will think you're 10 years old.

Lying flat refers to a defeatist lifestyle in Chin, where people stop working, desiring material acquisition, and tap out of any social life – sometimes for good. By lying flat, young people are participating in a kind of non-cooperation movement, rather than surrendering themselves to China's oppressive work culture where they see little hope of social mobility.