Where does “за́втра” come from?

за́втра (Russian) comes from Old East Slavic завътра, from Proto-Slavic *za utra.

Ancestry of “за́втра”, step by step

Step	Language	Word	Meaning
1	Old East Slavic	завътра	—
2	Proto-Slavic	*za utra	—

за́втра — Etymology, Origin & Word History | Etymology Explorer