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used as a polite way of addressing a man whose name you do not know, for example in a shop or restaurant （对不认识的男性的称呼或对男性的尊称）先生 Good morning, sir. Can I help you? 早上好，先生。您要点什么？ Are you ready to order, sir? 先生，可以点菜了吗？ ‘Thank you very much.’ ‘You're welcome, sir. Have a nice day.’ “多谢。” “不客气，先生。祝您愉快。”
- Dear Sir/Sirs
used at the beginning of a formal business letter when you do not know the name of the man or people that you are dealing with （正式信函中对不知其名的男性收信人的称呼）先生，阁下 Dear Sir/Sirs 亲爱的先生／诸位先生 Dear Sir or Madam 亲爱的先生或女士
- Sir
a title that is used before the first name of a man who has received one of the highest British honours (= a knight), or before the first name of a baronet （贵族头衔，用于爵士或准男爵的名字或姓名前面）爵士 -
used as a form of address to a man in a position of authority, especially in the armed forces （中小学生对男教师的称呼）先生，老师 - (British English)
used as a form of address by children in school to a male teacher （中小学生对男教师的称呼）先生，老师
Word OriginMiddle English: reduced form of sire.
Idioms
no sir! | no siree!
yes sir! | yes siree!