How did sign @ derived?

Answer 1

We were taught in school that it was called an apestaart - (Dutch for "monkey's tail")

There are 2 main explanations as to how it came about :

Before it became the standard symbol for e-mail, the @ symbol was typically used to indicate the cost or weight of something. For example, if you bought five oranges for $1.25 each, you might write it as 5 oranges @ $1.25 ea. It is still used in this manner on a variety of forms and invoices around the world.
The actual origin of the symbol is uncertain. It was used by monks making copies of books before the invention of the printing press. Since every word had to be painstakingly transcribed by hand for each copy of a book, the monks that performed the copying duties looked for ways to reduce the number of individual strokes per word for common words. So, the word at became a single stroke of the pen as @ instead of three strokes. While it doesn't seem like much today, it made a huge difference to the men who spent their lives copying manuscripts!

Another origin tale states that the @ symbol was used as an abbreviation for the word amphora, which was the unit of measurement used to determine the amount held by the large terra cotta jars that were used to ship grain, spices and wine. Giorgio Stabile, an Italian scholar, discovered this use of the @ symbol in a letter written in 1536 by a Florentine trader named Francesco Lapi. It seems likely that some industrious trader saw the @ symbol in a book transcribed by monks using the symbol and appropriated it for use as the amphora abbreviation. This would also explain why it became common to use the symbol in relation to quantities of something.

http://computer.howstuffworks.com/question507.htm

Answer 2

This sign is know as "The commercial 'at' " because it has been used in commerce to describe the price of articles for sale. For example you could say 'Apples for sale @ £2.00 a dozen.' Although used in other context's it is only meant to be used in this way so writing 'I will see you @ eight o'clock.' is, technically wrong

Answer 3

It appears to have been derived around the 6th or 7th century by scribes writing in Latin. They used it to symbolize the word for 'at' (towards, as is 'pointed at') which in Latin is 'ad'.

But no one knows for sure why...