A fun look at the origin and meaning of "ruhig" and why the sentence "Sei ruhig laut." actually makes sense. Also: die Ruhe and German for "Leave me alone!"
ruhig, die Ruhe, still, leise
The core idea of this root was:
The root is only mentioned by German etymological sources. The English ones that I checked do not mention it.
According to German sources, it is the root of the word rest in the sense of repose, but NOT of the word rest in the sense of leftover, which is from a different root apparently.
Doesn’t matter for us learners though.
If we think of it as “remaining still”, we can make sense of all the relatives.