Where does “肥立つ” come from?

肥立つ (Japanese) comes from Japanese 日, from Old Japanese 日, from Proto-Japonic *pi.

Ancestry of “肥立つ”, step by step

Step	Language	Word	Meaning
1	Japanese	日	Short for 日曜日 : Sunday; the sun; sunlight,...
2	Old Japanese	日	the sun; sunlight, sunshine; daytime, daylight
3	Proto-Japonic	*pi	—

肥立つ — Etymology, Origin & Word History | Etymology Explorer