Where does “ùstrajati” come from?

ùstrajati (Serbo-Croatian) comes from Serbo-Croatian uz, from Proto-Slavic vъz, from Proto-Balto-Slavic uź — on, at; in return for.

Ancestry of “ùstrajati”, step by step

Step	Language	Word	Meaning
1	Serbo-Croatian	uz	up, upward; next to, beside, alongside, by; with,...
2	Proto-Slavic	vъz	instead of, in return for; up; by, beside
3	Proto-Balto-Slavic	uź	on, at; in return for