The letter @ was used in the Middle Ages by monk scribes as an abbreviation for the sound "ad". They merged the "a" and the "d" to create a letter in this style.
Then, over time, the bar of the "d" was removed to give @.
Today, we say "at sign" because when the printer was invented, the printing characters were placed so that the lowercase letters were at the top of the table and the uppercase letters and special characters were at the bottom. The at sign was therefore at the bottom of the table and, to find it, printers called this letter: a-round-down. Because it was at the bottom of the table and there was a round around the "a". Over time, this name evolved to become "at sign.