"Shut up" is not a polite expression.

“住嘴”不是一个有礼貌的词语。

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"Shut up" is not a polite expression.

“住口”不是有礼貌的话。

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Polite language in English and Chinese involved many areas, and with many expression means, here just name a few common examples for reference.

英汉语中的礼貌语言涉及面广，表现手法众多，这里仅举一些常见例子以供参考。

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By contrast, a standard social smile, which is sometimes called a Pan Am smile after the polite expression the former airline's stewardesses used to greet passengers, activates only the muscles around the mouth.

WSJ: Stress-Busting Smiles