Where does “st + ipsu” come from?

st + ipsu (Vulgar Latin) comes from Latin ipsum, from Latin ipse, from Proto-Italic *e(s)peso.

Ancestry of “st + ipsu”, step by step

Step	Language	Word	Meaning
1	Latin	ipsum	accusative masculine singular of ipse; nominative...
2	Latin	ipse	himself, herself, itself, the very, the actual;...
3	Proto-Italic	*e(s)peso	—

st + ipsu — Etymology, Origin & Word History | Etymology Explorer