It seems to me it is very context dependent. I would say "OK, I'll shut up" in a couple of different situations. If I am in a discussion with someone and at a certain point it becomes clear they know much more about the subject that I do, I would say that as a way of saying "I should be quiet, because I have less to contribute than you do." Sometimes in a discussion in might become clear that the other person is tired of the topic and so the phrase is a way of saying "Let's change the topic because you don't seem to be enjoying this anymore." I don't see it as rude necessarily. But telling someone else to shut up is very rude and is best never done. I hope that helps.

You have asked: Does it show some kind of anger? Yes, most likely this is a reflection of an angry feeling. Another possibility, is that the person tried to be funny or sarcastic.

When someone asks you (or you volunteer to) do that, it is done out of irritation/anno yance/anger. So, yes, it is not said in polite conversations or conversations that are going well .

Hi Sycamor, It's kind of an odd thing to say if you thought the conversation was going well. Typically people say this if the other person has made it clear that they're annoyed at either what they are saying or how much they are talking. OR if the person who is talking feels like they have been dominating the conversation or talking too much, they might just say it spontaneously. Maybe your friend felt like that? I'm not sure.

Yes. Haha.. Message me if you want to practice English. I can help