In Chinese culture, where maintaining surface harmony (面子, miànzi) is often prioritized, communication can be indirect. The ability to “read the air” and understand unspoken intentions is a valuable social skill. The idiom `别有用心` is a tool for articulating the suspicion that someone is violating this social contract by feigning sincerity for personal gain. A useful Western comparison is the phrase “to have an ulterior motive” or “to have an axe to grind.” However, `别有用心` is arguably more integrated into everyday language. While “ulterior motive” sounds a bit formal or even clinical in English, `别有用心` can be used in casual gossip, serious business negotiations, and political commentary alike. It reflects a cultural vigilance towards potential deceit that may be more pronounced than in more direct, “say what you mean” Western cultures. It’s a warning label you apply to someone's behavior when you sense their kindness is a Trojan horse.
`别有用心` is a versatile idiom used in a wide range of contexts.
The connotation is overwhelmingly negative. It is an accusation of insincerity and hidden, selfish intent. You would never use it to describe a positive surprise, like planning a secret birthday party.