Abracadabra is a word famously used by magicians but which started out as a cabalistic or mystical charm for curing various ailments, including toothache and fever.
It was first mentioned in a poem called Praecepta de Medicina by the Gnostic physician Quintus Severus Sammonicus in the 2nd century AD. Sammonicus instructed that the letters be written on parchment in the form of a triangle. This was to be folded into the shape of a cross, worn for nine days suspended from the neck, and, before sunrise, cast behind the patient into a stream running eastward. It was also a popular remedy in the Middle Ages. During the Great Plague, around 1665, large numbers of these amulets were worn as safeguards against infection.
The origin of the word itself is uncertain. One theory is that it is based on Abrasax, the name of an Egyptian deity.