The symbol we know as the ampersand first appeared in some graffiti on a Pompeian wall around the first century A.D. It wasn’t called an “ampersand” at the time—it was just a ligature of the cursive letters “E” and “T” forming the Latin word et, which means “and.” (This is why “etc.” is sometimes written “&c”.)
At first, & had competition for use, as shorthand et—the “Tironian et” (⁊)—was created several hundred years before as part of Cicero’s secretary Tiro’s extensive shorthand system, thenotae Tironianae. But although it persisted into the Middle Ages, eventually the entire notae Tironianae fell out of use, leaving & to evolve and spread along with the language.
By the early nineteenth century, & was the 27th letter in the alphabet, coming right after Z. Without a title yet, it was still read as just “and,” which made reciting the end of the alphabet a little confusing—”X, Y, Z and and.” Kids starting inserting the phrase “and per se and” to distinguish it, and over time, this all got blended together to sound more and more like “ampersand.” The mondegreen name for the centuries old symbol first appeared in the dictionary in 1837.