Where does “загнуть” come from?

загнуть (Russian) comes from Russian гнуть, from Proto-Slavic gъnǫti — Alternative form of *gъbnǫti.

Ancestry of “загнуть”, step by step

Step	Language	Word	Meaning
1	Russian	гнуть	to bend, to curve; to drive at, to aim at
2	Proto-Slavic	gъnǫti	Alternative form of *gъbnǫti

загнуть — Etymology, Origin & Word History | Etymology Explorer