Where does “蝿毒草” come from?

蝿毒草 (Japanese) comes from Japanese 毒草, from Middle Chinese 毒草.

蝿毒草 (Japanese): lopseed

Definitions

Step	Language	Word	Meaning
1	Japanese	毒草	poisonous plant
2	Middle Chinese	毒草	—

蝿毒草 — Etymology, Origin & Word History | Etymology Explorer