In Chinese culture, which traditionally places a high value on diligence (勤奋 - qínfèn), ambition, and striving (奋斗 - fèndòu) for the sake of one's family and future, 混日子 is seen as a significant personal failing. It represents the opposite of the ideal worker or student. To accuse someone of 混日子 is to criticize their lack of responsibility and their waste of precious time and potential. A Western comparison might be “coasting” or “phoning it in,” but 混日子 often carries a heavier, more existential weight. It's less about a cheeky or defiant “slacker” attitude and more about a quiet, resigned sense of purposelessness. In recent years, the term has become closely associated with the social phenomenon of “lying flat” (躺平 - tǎng píng). While 躺平 is a conscious philosophy of opting out of the societal rat race due to immense pressure, 混日子 is often the day-to-day action or result of that philosophy. Someone who has decided to “lie flat” will likely spend their workdays in a state of 混日子.
混日子 is a highly informal and colloquial term used in everyday conversation.
The connotation is almost always negative. It can be used to criticize others, but it's also frequently used in a self-deprecating way to express dissatisfaction with one's own situation (e.g., “I feel like I'm just coasting at this dead-end job”).