What does the kettle mean? It seems a little abusive.

I think you can understand this from two levels:

Let's first describe a person who dares to say anything without opening the door, like a big teapot boiling water, which will be poured out as soon as it tilts.

Second, the current kettle has a whistle, and when the water boils, it will bark. Describe some people who are dying and don't know how to worry. Like a kettle, my ass is burning red and I'm in the mood to whistle.