Where does “син҄ь” come from?

син҄ь (Old Church Slavonic) comes from Proto-Slavic siňь — blue, bluish, glaucous; gray-blue, gray.

Ancestry of “син҄ь”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	siňь	blue, bluish, glaucous; gray-blue, gray

син҄ь — Etymology, Origin & Word History | Etymology Explorer