If you see something on your friend’s teeth and want to save him embarrassment, you might want to politely suggest he shut his mouth. You might think to say in a hushed voice that only he can hear:
shut, close + mouth, lips
You might try to say it in a very kind tone of voice, but it still might just come off as being rude or sarcastic unless you at least add the suggestion sound particle
You (should maybe) close your mouth.
is more like saying, “you should really consider closing your mouth right now”. However, it could be understood as, “you should really shut up,” partly because the 吧 might be misunderstood as 巴, which just means mouth. However, this “ba”, should be said with a first instead of a neutral tone, though.
Also, a lot can depend on your tone of voice. You can say something more inherently polite, such as
(ㄅㄧˋ ㄕㄤˋ)(ㄋㄧˇ ㄉㄜ˙)(ㄗㄨㄟˇ ㄅㄚ˙)
(bì shàng)(nǐ de.)(zuǐ ba.)
(to close, to shut up)(your)(mouth)
Close your mouth.
in a snarky tone and undo all the politeness of the words.
Of course, you can add other words to the sentence to make the tone of the suggestion more clear.
(ㄒㄧㄢˋ ㄗㄞˋˋ)(ㄋㄧˇ)(ㄎㄜˇ ㄧˇ)(ㄅㄧˋ ㄕㄤˋ)(ㄋㄧˇ ㄉㄜ˙)(ㄗㄨㄟˇ ㄅㄚ˙)
(xiàn zài)(nǐ)(kě yě)(bì shàng)(nǐ de.)(zuǐ ba.)
(now)(you)(may, possible)(to close)(your)(mouth)
You may close your mouth…
This is something a dentist might say to a patient or a make-up artist might say to an actor.
If you put
(you)(to give, for the benefit of)(me)
For my sake, ______________!
in front of
you are sending the distinct message that you are fed up with someone. It is the same as rudely or in anger saying, “Shut up!” in English.
For my sake, (just) shut up!
is not something you say to someone you ever want a good relationship with. 🤐
Like in English, context and tone are crucial to the final understanding of what and how something is being said. 閉嘴 could be said in an embarrassed or surprised way that is not rude.
So, a little bit of care about telling someone 閉嘴 will go a long way.