Here's why:
* Informal: It's not the kind of language you would use in a formal setting like a job interview or a professional meeting.
* Rude: The phrase is a direct command that can be interpreted as aggressive or dismissive. It implies that the speaker doesn't value the other person's opinion or right to speak.
More polite alternatives:
* "Please be quiet."
* "Let's keep this confidential."
* "I'd appreciate it if you wouldn't say anything about this."
The choice of language depends on the context and the relationship between the speaker and the listener.