Where does “бесконачан” come from?

бесконачан (Serbo-Croatian) comes from Serbo-Croatian без, from Proto-Slavic bez, from Proto-Indo-European *bʰeǵʰs.

Ancestry of “бесконачан”, step by step

Step	Language	Word	Meaning
1	Serbo-Croatian	без	without, excluding, not counting; linen, canvas;...
2	Proto-Slavic	bez	without
3	Proto-Indo-European	*bʰeǵʰs	—

бесконачан — Etymology, Origin & Word History | Etymology Explorer