Where does “sisältää” come from?

sisältää (Finnish) comes from Finnish sisä-, from Proto-Finnic sicä, from Proto-Uralic śićä.

sisältää (Finnish): To include; to contain; To involve

Definitions

Step	Language	Word	Meaning
1	Finnish	sisä-	inner, internal, interior, inward; indoor; inland
2	Proto-Finnic	sicä	—
3	Proto-Uralic	śićä	—

sisältää — Etymology, Origin & Word History | Etymology Explorer