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N-VOC
(對不知其姓名的男士或上級的尊稱)先生，長官
People sometimes say sir as a very formal and polite way of addressing a man whose name they do not know or a man of superior rank. For example, a shop assistant might address a male customer as sir .
例句
Excuse me sir, but would you mind telling me what sort of car that is?...
先生，請您告訴我那是什么類型的車好嗎？
Good afternoon to you, sir.
下午好，先生。
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N-TITLE
(用于爵士或準男爵的姓名前)爵士
Sir is the title used in front of the name of a knight or baronet.
例句
She introduced me to Sir Tobias and Lady Clarke.
她把我引見給了托拜厄斯爵士和克拉克夫人。
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CONVENTION
(正式信函或商務信函開頭的稱呼語)先生，閣下(寫信給某組織時使用 Dear sirs)
You use the expression Dear sir at the beginning of a formal letter or a business letter when you are writing to a man. You use Dear sirs when you are writing to an organization.
例句
Dear Sir, Your letter of the 9th October has been referred to us.
尊敬的先生，您 10 月 9 日的來信已轉交我處。