Where does “寝ぬ” come from?

寝ぬ (Japanese) comes from Japanese 寝, from Middle Chinese 寢.

寝ぬ (Japanese): to sleep

Definitions

Step	Language	Word	Meaning
1	Japanese	寝	sleep, sleeping; to sleep
2	Middle Chinese	寢	—

寝ぬ — Etymology, Origin & Word History | Etymology Explorer