Where does “пу̑ст” come from?

пу̑ст (Serbo-Croatian) comes from Proto-Slavic pȗstъ — desolate; empty.

Ancestry of “пу̑ст”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	pȗstъ	desolate; empty

пу̑ст — Etymology, Origin & Word History | Etymology Explorer