Found this on my dictionary yesterday.
This strange punctuation mark has a fascinating past. The ampersand emerged over 2,000 years ago as the Latin word et meaning “and.” The cursive writing of Latin scribes often connected the “e” and “t,” giving rise to the shape of the ampersand. The name did not appear until the 1830s when “&” was the 27th letter of the English alphabet. The mark concluded the alphabet with “X, Y, Z, and per se and” with “and per se” meaning “and by itself.” This final phrase was slurred by English school children during recitation and reborn as “ampersand.”