The characters combine to create a vivid image: “a dead/rigid nature that will not change.” It suggests that the person's flaws are not just temporary behaviors but are part of their unalterable core identity, making any attempt at correction futile.
`死性不改` reflects a certain fatalism in Chinese culture regarding human nature (`人性, rénxìng`). While self-cultivation (`修身, xiūshēn`) is a core Confucian value, this idiom represents the pessimistic view that some people's fundamental character (`本性, běnxìng`) is simply beyond repair. In Western culture, there's often a strong belief in the ability to “reinvent oneself” or have a “complete turnaround.” Concepts like the “growth mindset” are popular. `死性不改` stands in contrast to this, serving as a judgment that someone has exhausted all chances for reform. It's harsher than the English phrase “set in one's ways,” which can describe benign habits of an older person. `死性不改` almost always refers to a significant moral or personal failing, like dishonesty, laziness, or addiction, and implies that the person is a lost cause. It's a statement of giving up on someone.
This idiom is common in everyday conversation, often used to express frustration or exasperation. It is almost always negative and critical.