It's considered rude and is often used in a humorous or exasperated way, especially among friends. It's not something you'd typically say to someone you don't know well or in a formal setting.
Here are some examples of how it might be used:
* "Right, belt up, I'm trying to watch this!"
* "Can you belt up for five minutes and let me finish this?"
* "You're going on and on, just belt up already!"
It's worth noting that "belt up" is considered more of a colloquialism than a true slang term, and it is often used interchangeably with "shut up".