Where does “запахнуть” come from?

запахнуть (Russian) comes from Russian па́хнуть, from Proto-Slavic paxnǫti — to smell of.

Ancestry of “запахнуть”, step by step

Step	Language	Word	Meaning
1	Russian	па́хнуть	—
2	Proto-Slavic	paxnǫti	to smell of

запахнуть — Etymology, Origin & Word History | Etymology Explorer