When combined, “不-知-好-歹” literally means “not-know-good-(from)-bad”. This paints a vivid picture of a person who is so clueless or stubborn that they cannot differentiate between what will help them and what will harm them.
`不知好歹` is more than just a personal insult; it's deeply tied to Chinese social norms emphasizing hierarchy, respect for elders, and the importance of “goodwill” or `人情 (rénqíng)`. It's most often used by someone in a position of seniority (a parent, teacher, boss, or elder) towards someone junior who is defying their guidance. The senior person believes they are offering valuable wisdom born from experience, and the junior's rejection is seen not just as disagreement, but as a foolish and unappreciative act. A useful Western parallel is the sentiment behind “don't bite the hand that feeds you,” but with an added layer of frustration at the other person's foolishness. Imagine a parent forbidding their teenager from hanging out with a friend they know is a bad influence. The teenager protests, claiming the parent is being unfair. The parent, exasperated, might sigh and say, “你真是不知好歹!” (“You really don't know what's good for you!”). The phrase implies: “I am doing this for your own good, and you lack the wisdom to see it.” This reflects a cultural perspective where accepting advice from seniors is often seen as a sign of wisdom and respect, while rejecting it can be perceived as arrogant and `不知好歹`.
`不知好歹` is a strong criticism and carries a heavily negative connotation. It's almost always used to express frustration, disappointment, or contempt.