I've found the following part on gnu.org (https://www.gnu.org/philosophy/open-source-misses-the-point.en.html):
However, the obvious meaning for the expression “open source software” is “You can look at the source code.” Indeed, most people seem to misunderstand “open source software” that way. (The clear term for that meaning is “source available.”)
They justify it as follows:
Why do people misunderstand it that way? Because that is the natural meaning of the words “open source.”
When I tried to search what a natural meaning is, I found an article that I don't fully understand. It is https://www.open.edu/openlearn/history-the-arts/philosophy/language-and-thought-introducing-representation/content-section-2.3 and it first states that a natural meaning can't lie just after showing an example where it can (the spots can be fake) and later states that it can be 4 things, including being an evidence of that, although probably any meaning can be evidence (maybe someone concluded you have measles and wrote it, but for some reason had to do so using spots). So I don't understand that.
If I had to guess what “open source” meant, I would probably go with the opposite: it's ‛ready to accept changes into the source code’ (compare open-minded). Similarly, if I had to guess what free (as in freedom) software was, I would probably go with the opposite of what it actually means: I would think it means that the software (or its developer) has the freedoms (so the co-processor firmware is free in this sense) while it actually means that the users are free (or, as someone put it, “if the product is free, you're the product”).