Where does “nin” come from?

nin (Old Spanish) comes from Latin nec, from Latin neque, from Proto-Indo-European nekʷe, from Proto-Indo-European ne — not.

Ancestry of “nin”, step by step

Step	Language	Word	Meaning
1	Latin	nec	nor; and not, not; neither
2	Latin	neque	not; and not, also not
3	Proto-Indo-European	nekʷe	and not, neither, nor
4	Proto-Indo-European	ne	not