Where does “निश्” come from?

निश् (Sanskrit) comes from Sanskrit नि-, from Proto-Indo-Iranian ní, from Proto-Indo-European ni — down.

Ancestry of “निश्”, step by step

Step	Language	Word	Meaning
1	Sanskrit	नि-	into; down; without, free of
2	Proto-Indo-Iranian	ní	down, downwards; in, into
3	Proto-Indo-European	ni	down