In the 1400s and 1500s, it was common in English to distinguish single-letter words like A, I, and the vocative O (“O captain!”) from letters by specifying “I per se, I” or translated from the customary Latin “I by itself, I”. In addition, the ligature for Et, Latin for “and”, was written so frequently that it was stylized as a single separate character, &.

By 1796, the combination of both of those trends had gotten so common that people started mockingly writing out the spoken “and per se, and” as “ampersand”, which stuck as the name of the & symbol itself.