'亏心' literally means 'to lose heart' or 'to damage the heart,' but idiomatically it describes a deep, internal sense of guilt or moral unease—when one’s conscience feels compromised after doing something dishonest, unfair, or unethical. The character 亏 (kuī) conveys loss, shortfall, or harm, while 心 (xīn) means 'heart' but here symbolizes one’s inner moral compass or conscience. Together, they evoke the feeling that one has injured their own integrity.

This term is commonly used in spoken and written Chinese to express self-reproach or to accuse others of lacking moral grounding. It often appears in moral discussions, storytelling, or everyday reflection—e.g., when someone justifies an action by saying '我问心无愧' ('I have a clear conscience'), the implied contrast is with being 亏心. It carries a strong ethical weight and is rarely used lightly or humorously.