Where does “запалить” come from?

запалить (Russian) comes from Russian пали́ть, from Proto-Slavic pālìti.

Ancestry of “запалить”, step by step

Step	Language	Word	Meaning
1	Russian	пали́ть	to fire, to burn; to burn, scorch
2	Proto-Slavic	pālìti	—

запалить — Etymology, Origin & Word History | Etymology Explorer