Where does “spilg-” come from?

spilg- (Proto-Balto-Slavic) comes from Proto-Indo-European pel- — to cover, to wrap; skin, hide, cloth; to fold.

Ancestry of “spilg-”, step by step

Step	Language	Word	Meaning
1	Proto-Indo-European	pel-	to cover, to wrap; skin, hide, cloth; to fold

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