Where does “*ěsnostь” come from?

*ěsnostь (Proto-Slavic) comes from Proto-Slavic ěsnъ, from Proto-Slavic ěsknъ, from Proto-Balto-Slavic áiˀšknas.

Ancestry of “*ěsnostь”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	ěsnъ	clear, bright; clear, serene; clear, lucid,...
2	Proto-Slavic	ěsknъ	—
3	Proto-Balto-Slavic	áiˀšknas	—

*ěsnostь — Etymology, Origin & Word History | Etymology Explorer