I think perhaps arguing about the arbitrary nature of definitions will devolve into pedantry. While you are completely correct (e.g. the meaning of "toilet" over time). There is perhaps a better current arbitrary word (mouth hole sound) that implies something deeper. There are immutable essences (or platonic forms) that cannot change that we have been describing with different language since the very beginning. It kind of reminds me of that scene in the matrix where Neo meets the machine father with his daughter who tells him how "love" is just a word. What it implies is what matters. It doesn't matter how you describe it visually or audibly, if you are communicating with someone else, and they are trying to understand you, the reason they ever can is because something exists independent of either communicator, and both have recognized that thing and are willing to acknowledge that.
About the later part: yes, I actually agree!
If you have technical document or research paper there are definitions at the start which are defined for the scope of a document or a paper. Those are still arbitrary but at least they have some value.