The "at" Symbol

The earliest known example of @ is found in the 1345 Bulgarian translation of the 12th century Manasses Chronicle, which gives a brief synopsis of the history of the world up to the end of the 11th century. In it, @ was used as the symbol for “amin” (amen).

The Smithsonian website differs in the first use, but there's some interesting information in this piece:

The Accidental History of the @ Symbol