OriginProbably from Vulgar Latin *metipsimus, from Latin -met + ipse + -issimus. Compare Ladin medemo, Romansch medem, madem, Italian medesimo, Venetan medemo, medesino, Lombard medemm, Piedmontese midem, Romagnol medësum, French même, Portuguese mesmo, Spanish mismo.
Formsmidiesim(alternative)
Source: Wiktionary