Until as recently as the early 1900s, “&” was considered a letter of the alphabet and listed after Z in 27th position. To avoid confusion with the word “and”, anyone reciting the alphabet would add “per se” (“by itself”) to its name, so that the alphabet ended “X, Y, Z and per se &”. This final “and per se and” eventually ran together, and the “ampersand” was born.
From ‘A’ to ‘ampersand’, English is a wonderfully curious language; Guardian; 15/02/2014