Today I had a lot of fun with a group of mums and kids in a muddy obstacle race. I fell into the muddy pool while my mouth was opened and drank some muddy water by accident. Then I said to the other mum I better "keep my mouth shut" as I didn't want to drink anymore muddy water. The instructor then said I was rude by saying that.
Is it very rude and impolite to say that in that situation please?
The phrase keep your mouth shut could be used literally, as you have used it, to mean that you should physically close your mouth to keep something from getting into it. I can think of no situation where using the phrase as you have used it could be interpreted as you being rude.
The phrase can also be used to mean shut-up or don't speak. For example someone might say, "When we are in Mr. Bigg's office discussing big plans, you should keep your mouth shut." Used this way the phrase is stern at best and possibly rude.
Based on your experience it sounds like someone misinterpreted what you had said.
Answering from a US English perspective. I think what you said was misheard. I can't think of a situation where talking about keeping your own mouth shut would be rude. But telling someone else to keep their mouth shut would be considered rude.
Although I think the instructor may have misinterpreted, I can offer an alternative.
"keep one's mouth shut" usefully refers to speaking, especially when coupled with "keep".
I might have used the phrase "Next time I'll remember to keep my mouth closed!"
Or even safer, "Next time I'll remember not to leave my mouth open!"
I agree that the instructor probably misheard! But some context might be helpful.
"Keep your mouth shut" is a very rude, often aggressive or threatening way of telling someone to stop talking, or to not say anything (maybe about a particular subject). You could say it to a friend in a non-serious way, but you wouldn't say it to someone you were being polite to. If the instructor thought you said this to somebody, they definitely might criticise you for it!
"I'll keep my mouth shut" isn't about another person, so it's less confrontational - but it isn't exactly polite. It can be a bit like telling yourself to shut up, and some people might be sensitive to that kind of language. It can also imply that someone is ordering you to keep quiet - "I'll keep my mouth shut" can sound like someone is forcing that on you, or you've offended them, or you're implying there's something bad you're promising not to mention.
So I agree with the other posters who think they'd probably say "I'll keep my mouth closed" - it doesn't have any of that baggage or negative association. "Mouth closed" usually just means "not open", but "mouth shut" usually means "silent"
