Where does “ку̏к” come from?

ку̏к (Serbo-Croatian) comes from Proto-Slavic kъlkъ, from Proto-Balto-Slavic kaukāˀ, from Proto-Indo-European klHk(ᵂ)-(n)-.

Ancestry of “ку̏к”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	kъlkъ	—
2	Proto-Balto-Slavic	kaukāˀ	—
3	Proto-Indo-European	klHk(ᵂ)-(n)-	—

ку̏к — Etymology, Origin & Word History | Etymology Explorer