12th cent. From Late Latin per hoc "therefore" (lit. "through that"). Used in the sense of "but" or "yet" in Latin in Iberia since the 6th cent. For the etymology of per, see por. Hoc is the neuter of hic; for the etymology of hic, see ahí.
Bilbao peró reflects an alternative accentuation in Vulgar Latin of per hóc instead of pér hoc
Ladino (Marruecos) peró, Catalan però (both from per hóc)