Where does “-ьнъ” come from?

-ьнъ (Old East Slavic) comes from Proto-Slavic -ьnъ — -ish, -ic, -al, -ed; From adjectives.

Ancestry of “-ьнъ”, step by step

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

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