“Am·per·sand [ä'mper_sänd] (1837), n. A character or symbol (&) for and. From the practice of British schoolchildren, when reciting the alphabet, of giving all twenty-six letters plus the & sign, which they describe thus: ‘X, Y, Zed and per se [by itself] and’. The last phrase was gradually slurred into ampersand.”

Morris Dictionary of Word and Phrase Origins