It is now commonly used as an incantation by magicians. In ancient times, however, the word was taken much more seriously as an incantation to be used as a cure against fevers and inflammations. The first known mention was in De Medicina Praecepta by Serenus Sammonicus, physician to the Roman emperor Caracalla, who prescribed that the sufferer from the disease wear an amulet containing the word written in the form of an inverted cone:
A B R A C A D A B R A
A B R A C A D A B R
A B R A C A D A B
A B R A C A D A
A B R A C A D
A B R A C A
A B R A C
A B R A
A B R
This, he explained, diminishes the hold of the spirit of the disease over the patient. Other Roman emperors, including Geta and Alexander Severus, were followers of the medical teachings of Serenus Sammonicus and are likely to have used the incantation as well.
There is also the view that Abracadabra derives from the Hebrew, ha-brachah, meaning "the blessing" (used in this sense as a euphemism for "the curse") and dabra, an Aramaic form of the Hebrew word dever, meaning "pestilence." They point to a similar kabbalistic cure for blindness, in which the name of Shabriri, the demon of blindness, is similarly diminished. Other scholars are skeptical of this origin and claim that the idea of diminishing the power of demons was common throughout the ancient world, and that Abracadabra was simply the name of one such demon.
Some point to the Hebrew words ab ("father"), ben ("son"), and ruach hacadosch ("holy spirit").
Some have argued that the term may come from the Arabic Abra Kadabra, meaning 'let the things be destroyed' or from the Aramaic abhadda kedhabhra, meaning 'disappear like this word'. Rather than being used as a curse, the Aramaic phrase is believed to have been used as a means of treating illness.
It has also been claimed that the word comes Abraxas, a Gnostic word for God (the source of 365 emanations, apparently the Greek letters for Abraxas add up to 365 when deciphered according to numerological methods).
See also: Hocus Pocus, presto and Avada Kedavra (a Harry Potter reference).
for a variant, see Avada Kedavara