Where does “kur-” come from?

kur- (Proto-Balto-Slavic) comes from Proto-Indo-European kr̥-, from Proto-Indo-European ker- — army.

Ancestry of “kur-”, step by step

Step	Language	Word	Meaning
1	Proto-Indo-European	kr̥-	to turn, to bend
2	Proto-Indo-European	ker-	army