1837, contraction of and per se and, meaning "(the character) '&' by itself is 'and' " (a hybrid phrase, partly in Latin, partly in English). The symbol is based on the Latin word et "and," and comes from an old Roman system of shorthand signs (ligatures), attested in Pompeiian graffiti, but not (as sometimes stated) from the Tironian Notes, which was a different form of shorthand, probably invented by Cicero's companion Marcus Tullius Tiro, which used a different symbol, something like a reversed capital gamma, to indicate et.
This Tironian symbol was maintained by some medieval scribes, including Anglo-Saxon chroniclers, who sprinkled their works with a symbol like a numeral 7 to indicate the word and. In old schoolbooks the ampersand was printed at the end of the alphabet and thus by 1880s had acquired a slang sense of "posterior, rear end, hindquarters."