Where does “mesmu” come from?

mesmu (Asturian) comes from Vulgar Latin metipsimus, from Latin ipse, from Proto-Italic *e(s)peso.

mesmu (Asturian): same

Definitions

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

mesmu — Etymology, Origin & Word History | Etymology Explorer