Even some thousand years ago, you could well have acquired koschei for personal use, even if you were not Marya Morevna or Vasilisa the Wise.
In "The Lay of Igor's Regiment" "koshchei" is mentioned twice, and once the adjective "koshchievo" is encountered.
All three situations are very curious and ambiguous.
Situation 1. Mysterious "price list"
Nogata and Rezana are monetary units with low purchasing power. In this line, the author dreams of reducing the cost of some chaga and koshchei.
Situation 2. Swearing
Here everything is more or less clear: "koshchey" acts as an offensive curse.
Situation 3. Mysterious adjective
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That is, for some reason, the prince changed the gold-embroidered saddle for a kind of "koschievo saddle". Where did it come from?
The most reasonable version of interpretation comes down to the fact that the word "koshchey" came to us from Eastern languages and began to denote a captive, a slave. In the Russian language, other names for slaves also had an abusive connotation (remember how Shpak took offense at the word “slave” in “Ivan Vasilievich”).
Also Turkism "koshchy" could mean a special servant for caring for a horse.
Then in situation No. 1 we are talking about slaves: in the event of a victory there will be so many captives that they can be bought at dumping prices. And in situation No. 3, the prince, for some reason, sits in the servant's saddle.
Rezana is half a dirham. At this price, the author of * Words * dreams of acquiring * koshcheev *.
To whom it seems strange that the "Word" scolds Khan Konchak as a "filthy slave" (not by status somehow) - there is another version of the translation "nomad".
We add that the name "Koschey" is also found in birch bark letters.
For what reasons it "stuck" to the folklore character is very difficult to say. Perhaps because of the consonance with the word "bones" (or "sacrilege").
By the way, in the tale about Marya Morevna, Koschey is a prisoner, and, like a slave, he is kept in chains.
Interestingly, the word really turned out to be "immortal" - it has been living in the language for a thousand years under different guises, refusing to disappear.