Where does “脂” come from?

脂 (Chinese) comes from Proto-Sino-Tibetan tsil, from Proto-Finno-Ugric *siili.

Ancestry of “脂”, step by step

Step	Language	Word	Meaning
1	Proto-Sino-Tibetan	tsil	fat, grease; oil
2	Proto-Finno-Ugric	*siili	—