Where does “ipsas” come from?

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

Ancestry of “ipsas”, step by step

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

ipsas — Etymology, Origin & Word History | Etymology Explorer