Where does “nether” come from?

nether (Middle English) comes from Old English niþera, from Proto-Germanic niþeraz, from Proto-Indo-European h₁én — in.

Ancestry of “nether”, step by step

Step	Language	Word	Meaning
1	Old English	niþera	lower, lowest, under; inferior
2	Proto-Germanic	niþeraz	—
3	Proto-Indo-European	h₁én	in

nether — Etymology, Origin & Word History | Etymology Explorer