Where does “заваливать” come from?

заваливать (Russian) comes from Russian завали́ть, from Russian за́-, from Proto-Slavic za- — for.

Ancestry of “заваливать”, step by step

Step	Language	Word	Meaning
1	Russian	завали́ть	—
2	Russian	за́-	—
3	Proto-Slavic	za-	for

заваливать — Etymology, Origin & Word History | Etymology Explorer