Where does “ekster” come from?

ekster (Esperanto) comes from Latin extrā, from Latin exter, from Proto-Italic *eksteros.

ekster (Esperanto): outside

Definitions

Step	Language	Word	Meaning
1	Latin	extrā	—
2	Latin	exter	On the outside, outward, external, outer; Of...
3	Proto-Italic	*eksteros	—

ekster — Etymology, Origin & Word History | Etymology Explorer