"先生 (xiān sheng)", an age-old Chinese word, is definitely not just a simple Chinese word for "Mr.". In different dynasties in Chinese history, "先生 (xiān sheng)" was used to address people with different social status.
It writes in The Analects:
"If, when their elders have any troublesome affairs, the young take the toil of them, and if, when the young have wine and food, they set them before their elders, is this to be considered filial piety?"
"先生 (xiān sheng)" here refers to "father and elder brothers", and that's why James Legge, a British sinologist, translated "先生 (xiān sheng)" in this sentence into "elders".
The word "先生 (xiān sheng)" can also be found in The Mencius:
"He came to see Mencius, who said to him, 'Do you also come to see me?' Yo-chang replied, 'Master, why do you think so?'"
Here, "先生 (xiān sheng)" is used to address the learned respectfully, unrelated to the occupation or gender. So James Legge translated "先生 (xiān sheng)" here into "master".
《孟子》里也提到了"先生"一词："乐正子见孟子。孟子曰：子亦来见我乎？ 曰：先生何为出此言也？"这里"先生"是指有学问的人，更多体现的是一种敬重，并无职业和性别的指代性。因此，James Legge把先生译成了"master"。
Later, the word "先生 (xiān sheng)" was used to address teachers. After the Revolution of 1911, "先生 (xiān sheng)" was used to address teachers, educators and the learned. Any highly respected person, male or female, can be called "先生 (xiān sheng)" to show the speaker's respect.
Sine the 1980s, as a result of reform and opening-up, communication between strangers has became much more frequent. When meeting a man for the first time, one will address him "先生 (xiān sheng)". Then "先生 (xiān sheng)" has become a general appellation for men. In this sense, it is equivalent to "mister". For example:
A: Nice to meet you, Mr. Li. I am Chen Si from HR Dep..
B: Nice to meet you, Mr. Chen.
In addition, "先生 (xiān sheng)" can also be used to address one's husband. When a female introduces her husband, usually she will use "先生 (xiān sheng)". For example:
xiǎo lǐ, gēn nǐ jiè shào xià, zhè shì wǒ xiān sheng zhāng sì.
Lee, it's my husband Zhang Si.
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