Ampersand, meaning ‘and’, is a contraction of the hybrid Latin/English phrase ‘and per se and’, which was first recorded in the 1830s. ‘And per se and’ sounds like gibberish but, according to the OED, it means ‘the character & by itself is and’; it’s a way of highlighting that the symbol stands for the word.
Another slight variant of this origin is that in 19th century schools, & was added to the end of the alphabet. When schoolchildren recited the alphabet, they preceded characters that could stand alone as words (including I and a) with per se, meaning ‘by itself’. As & came at the end of the alphabet, the recitation would finish with ‘x, y, z and per se and’.
The origin of the symbol itself is in a Roman system of shorthand symbols, which can be seen in graffiti in Pompeii. The Latin word for and is et. Blend the two letters together and you get something that looks like &. It’s clearer in some fonts than others; try typing & in Trebuchet MS and see what it looks like.
Interestingly, there was another system of Roman shorthand called Tironian notes, said to have been invented by Cicero’s scribe, Marcus Tullius Tiro. The Tironian symbol for ‘and’ looked more like the number 7 and it was maintained by some medieval scribes. One theory has it that when the first keyboards were laid out, it was decided that & should be the capital version of 7 so that if the typist failed to hit shift and type the &, at least he would get the closest match to the Tironian ‘and’.