1. (对不知其姓名的男士或上级的尊称)先生，长官
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 .
e.g. Excuse me sir, but would you mind telling me what sort of car that is?...
先生，请您告诉我那是什么类型的车好吗？
e.g. Good afternoon to you, sir.
下午好，先生。
2. (用于爵士或准男爵的姓名前)爵士
Sir is the title used in front of the name of a knight or baronet.
e.g. She introduced me to Sir Tobias and Lady Clarke.
她把我引见给了托拜厄斯爵士和克拉克夫人。
3. (正式信函或商务信函开头的称呼语)先生，阁下(写信给某组织时使用 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.
e.g. Dear Sir, Your letter of the 9th October has been referred to us.
尊敬的先生，您 10 月 9 日的来信已转交我处。