sir
/sɜː(r)/(亦作Sir)
noun
1- used as a polite or respectful way of addressing a man, especially one in a position of authority先生; 长官（尤指对上级的尊称）：
excuse me, sir.
劳驾, 先生。
1.1
- used to address a man at the beginning of a formal or business letter先生, 阁下（正式或商业信函开头的称呼）：
Dear Sir.
亲爱的先生。
1.2
- used as a title before the forename of a knight or baronet爵士（用于骑士或准男爵教名之前的称号）。
词源
Middle English: reduced form of SIRE.