Where does “-ни́й” come from?

-ни́й (Ukrainian) comes from Proto-Slavic -ьnъ — -ish, -ic, -al, -ed; From adjectives.

Ancestry of “-ни́й”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	-ьnъ	-ish, -ic, -al, -ed; From adjectives

-ни́й — Etymology, Origin & Word History | Etymology Explorer