Where does “јак” come from?

јак (Serbo-Croatian) comes from Tibetan གཡག, from Proto-Sino-Tibetan g-jak ~ g-jaŋ — sheep, yak.

Ancestry of “јак”, step by step

Step	Language	Word	Meaning
1	Tibetan	གཡག	yak
2	Proto-Sino-Tibetan	g-jak ~ g-jaŋ	sheep, yak

јак — Etymology, Origin & Word History | Etymology Explorer