Many online dictionaries (dictionary.writtenchinese.com, yellowbridge.com, chine.in/mandarin/dictionnaire/, app.ninchanese.com/search, etc...) mention the meaning "actually" ("en réalité" for French dict chine.in) for this character.
But my Wenlin dict. and others (bishun.shufaji.com/, humanum.arts.cuhk.edu.hk//Lexis/lexi-mf/search.php?word=所, hanziyuan.net, etc...) do not mention this use !
Is that meaning really used in Modern Chinese nowadays ?
I couldn't recall any in either Modern or Literary Chinese. Consulting a bunch of authoritative dictionaries confirms it (see the entry screenshot in the next section).
I really recommend against those quoted dictionaries (except for the cuhk one). On the mainland, 《新华字典》is the dictionary for primary schools and one of the two dictionaries for middle and high schools with regard to Modern Chinese. It has an official bilingual version, in which both the explanations and examples are presented with English translations. The examples also come with pinyin. A paper copy is cheap, and a free text mdx version is easy to get by (but in this case you need to pay attention to the copyright issues in your regions). The entry for 所 is as follows:
The character 所 is created to mean 'sound of wood chopping'. This original usage was very rarely used; attestations are mainly found in《诗经》.
所 was loaned to denote the noun 'place' (corresponding to meanings 1,2,3 in 《新华词典》), which developed into a classifier (4) and a pronoun (5), and the pronoun further grammaticalized to a verb prefix (6). Since 《新华词典》 is at the entry level, it is ok to treat meanings 5 as auxiliaries, but it is actually a pronoun (see 王力 《汉语语法史》). The above are actively used in both Modern and Literary Chinese.
If we extend to Literary Chinese, we can find some other less common meanings, none of which are related to 'actually'.
王力《古漢語字典》
《辭源》
《漢語大詞典》
《教育部重編國語辭典》(https://dict.revised.moe.edu.tw/dictView.jsp?ID=10158&q=1&word=%E6%89%80)
REQUESTED BY PdotWang
王力 discussed 所 in Chapter 5 指示代词. Here I post the screenshots of relevant pages.
RESPONSE TO @faure
I checked wenlin, it's good to offer some translation of 说文. The etymology and glyph composition are concise and useful. But I am only able to consult one word without login and thus cannot assess its overall accuracy. I opened hanziyuan.net and recognize that guy. Then I watched one video from him about 帽. He actually writes 冒 wrong. The upper part must be wider than the lower. And his claim that 𨸏 is wall is not attested. Again, I don't know enough about him to assess the overall accuracy.
Check https://mdict.org/post/0005/ for 新华字典双语版. You also need a software (free or open-source ones available for all platforms) for the mdx format.
Whether "所" means "actually"? Is that meaning really used in Modern Chinese?
My answer to your question is No. The word "所" does not mean "actually" in any sense.
The word "所" has the following meanings,
(1) A noun = a place. For example, 厕所， 诊所。
(2) A pronoun = that (or which) in a clause. For example,
他所说的并不真实。= What he said is not true.
(3) A measuring word (or a classifier) = a number of places. For example,
县城有两所高中。= There are two high schools in town. (no need to translate 所 into English).
(4) Used in a passive sentence as 被--所--. For example, 他被生活所困。= He is confined by making living.
I found an example that translates the word "所" into "actually". But I think it is a misunderstanding of the usage of the word "所". I would like to give you a scenario to explain. For example,
(1) 虽然这本字典有一万个词汇，但是实际上学生所需要学习的只有三千个。
(2) 虽然这本字典有一万个词汇，但是学生所需要学习的只有三千个。
(1) and (2) have the same meaning = Even though there are ten thousand words in this dictionary, the vocabulary that a student needs to learn is actually only three thousands.
As you can see that the meaning of actually is indicated by 实际上. In (2), the word 实际上 is omitted, but it still has the same meaning.
The meaning of actually is not from the word 所.
Are your dictionary references listing it as a vocabulary definition, or as a general character meaning? This is a very important distinction to make and what I believe may be the root of your confusion. A characters individual meanings does not automatically overlap with its use as vocab in modern mandarin chinese.
None of my dictionaries listed actually as a definition of 所. All of them listed it as a character meaning... among an average dozen character meanings of all sorts. For record quite a few of these had no modern vocabulary relevancy or were purely grammatical etc.
Baidu Encyclopedia does mention this usage, however as usual it shows very ancient sourcing for meaning usage of terms. I see no indication of it having modern use.
So I would say that the character 所 has relationship to the meaning actual/actually . However the vocabulary 所 does not have this definition in modern mandarin chinese, except maybe in some set phrases/chengyu leftover from older chinese etc.
(p.s. cantonese I think has much closer use to actual/actually , but still not as an... actual vocab term :p)
