The characters combine literally and powerfully: “Not (不) hit (打), yet oneself (自) confesses (招).” The meaning is derived directly from this vivid image of a confession given before the punishment or pressure even begins.
`不打自招` is deeply rooted in a cultural understanding of human psychology, particularly the concept of a guilty conscience (`做贼心虚 - zuò zéi xīn xū`). Chinese stories, from ancient legal cases to modern TV dramas, often feature characters whose guilt is so overwhelming that it inevitably surfaces through a slip of the tongue or a nervous action. This idiom captures the belief that truth, especially regarding wrongdoing, has a way of coming out, and that often, the guilty party is their own undoing. A useful Western comparison is a “Freudian slip,” where someone accidentally says what they are subconsciously thinking. However, there's a key difference. A Freudian slip is about revealing a hidden desire or thought. `不打自招` is almost always about revealing guilt or a hidden action. It carries a much stronger implication of wrongdoing. While you could have a Freudian slip about being in love with someone, you would `不打自招` about having stolen the last cookie. The focus is on self-incrimination.
This idiom is common in both spoken and written Chinese. It's versatile and can be used in serious, humorous, or sarcastic contexts.