Where does “ipsative” come from?

ipsative (English) comes from Latin ipse, from Proto-Italic *e(s)peso.

ipsative (English): denoting a measure that forces an individual to...

Definitions

Step	Language	Word	Meaning
1	Latin	ipse	himself, herself, itself, the very, the actual;...
2	Proto-Italic	*e(s)peso	—

ipsative — Etymology, Origin & Word History | Etymology Explorer