when was the symbol @ first used instead of the word at?

[Nigel]

[Miggy]

The character @, called the at sign or at site or at symbol, is an abbreviation of the word at or the phrase 'at the rate of'' in accounting and commercial invoices (e.g. "6 widgets @ 3p = 18p"). It has been used in this context for hundreds of years

Its most common modern use is in e-mail addresses, where it stands for "located at". Increasingly, @ is also used as a prefix to user names (e.g. "@username") on social websites such as Bebo and Twitter to denote a link, attribution or indirect reference.

The symbol was included on the keyboard of 1885 American Underwood, the first typewriter,and remained largely obscure until Raymond Tomlinson, an American programmer, used it in 1971 as the natural division within the first e-mail message sent.

[sneezewhiz]

IN an e-mail address, it goes back to 1971.

There's a fascinating Wikipedia article on it.

[cymry3jones]

There actually was life before the Internet. The @ symbol has been used for centuries:
Ten sheep @ £10 per sheep: £100.
Not long ago I sent this quote for a translation to an agency:
5000 words @ € .05 per word: € 250.--
The agency sent me an e-mail saying: You are not allowed to put an e-mail address in your quote.

[Laurence]

The symbol is a lower case "a", circled to show it is being used as an abbreviation. It began in early medieval Latin to be used for "ad" (to or at), which was often used commercially as in "12 pisces @ £1". (twelve fishes at one libra each)."
Its name, the "arrobase", is from the Arabic "ar-rub" meaning "one quarter", which was hispanised as "arroba" in Spanish and Portuguese, and used for a weight of one quarter (a fourth of a hundredweight): 11.5 kg in Spain, 14.7 kg in Portugal), and already by 1088, "@" was being used in Spain as an abbreviation for this weight. The abbreviation @ is still known as an "arroba" in Portuguese, but this became "arrobase" in French, and this seems to be the only name for it in English too (unless you are happy just to call it "the at sign.")
Its internet use dates from 1971.