Where does “天草” come from?

天草 (Chinese) comes from Japanese 天草, from Japanese 天, from Old Japanese 天, from Proto-Japonic *amay.

Ancestry of “天草”, step by step

StepLanguageWordMeaning
1Japanese天草any of the red algae of the genus "Gelidium";...
2Japanese天the sky, the heavens; the area above a kitchen...
3Old Japanese天sky, heavens
4Proto-Japonic*amay—
Every word from Proto-Japonic *amay →Every word from Old Japanese 天 →Every word from Japanese 天 →