And per se and

The ampersand -- & -- has an interesting history.

The ampersand can be traced back to the first century AD. It was originally a ligature of the letters E and T (”et” is Latin for and). If you look at the modern ampersand, you’ll likely still be able to see the E and T separately.

The first ampersands looked very much like the separate E and T combined, but as type developed over the next few centuries, it eventually became more stylized and less representative of its origins.

You can see the evolution of the ampersand below (1 is like the original Roman ligature, 2 and 3 are from the fourth century, and 4-6 are from the ninth century).

The modern ampersand has remained largely unchanged from the Carolignian ampersands developed in the ninth century.

And there's more:

The word ampersand is a corruption of the phrase "and per se ("through itself") and", meaning "and [the symbol which] by itself [is] and".

Traditionally, in English-speaking schools when reciting the alphabet, any letter that could also be used as a word in itself ("A," "I," and, at one point, "O") was preceded by the Latin expression "per se" (Latin for "by itself"). Also, it was common practice to add at the end of the alphabet the "&" sign, pronounced "and". Thus, the recitation of the alphabet would end in: "X, Y, Z and per se and." This last phrase was routinely slurred to "ampersand" and the term crept into common English usage by around 1837. 

And that's it.