Recently I started thinking about what an old equivalent of the word "серьёзный" could be, maybe being used in Kievan Rus or in general before the word came to the language. It is clearly a loanword; one compare serious in English and sérieux in French.
There are examples in other Slavic languages. In Serbian, for instance, "серьёзный" would be ozbiljan (comes from the verb zbiti se, related to Russian "сбыться"). Could there be an old analogue of this in Russian?
In general, my question would be whether at some point in time there existed a different word, maybe with a Slavic root, which was used to mean "серьёзный".