Where does “-чьнии” come from?

-чьнии (Old East Slavic) comes from Proto-Slavic -ťь — Alternative form of *-tь.

Ancestry of “-чьнии”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	-ťь	Alternative form of *-tь

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