Where does “до́лжный” come from?

до́лжный (Russian) comes from Proto-Slavic *dъlžьnъ, from Proto-Slavic dъlgъ, from Proto-Indo-European *dʰlegʰ-.

Ancestry of “до́лжный”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	*dъlžьnъ	—
2	Proto-Slavic	dъlgъ	debt
3	Proto-Indo-European	*dʰlegʰ-	—

до́лжный — Etymology, Origin & Word History | Etymology Explorer