As my latest infographic will tell you, the ampersand was once considered to be the twenty-seventh letter of the alphabet. It appeared right at the end, so when children sang the alphabet song, they would sing w, x, y, z, and per se &. It was fashionable in the olden days to use the Latin phrase per se, which meant "of itself", to distinguish any letter that could be used as a word itself to avoid making things confusing (so this was also done for A, I, and O). When children quickly sing songs, they tend to slur words, and over time, the words and per se and were slurred to ampersand, and that's how we got the term. The symbol itself is a ligature of the Latin word et, meaning "and", as the e and t fused in Roman cursive.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.