Abracadabra is of unknown origin, and its first occurrence is in the second century works of Serenus Sammonicus, according to the Oxford English Dictionary. Several folk etymologies are associated with the word: from phrases in Hebrew that mean "I will create as I speak", or Aramaic "I create like the word", to folk etymologies that point to similar words in Latin and Greek such as abraxas. According to the OED Online, "no documentation has been found to support any of the various conjectures."

Source : Wikipedia

Etymology Online links it to the Late Greek mystic word 'Abraxas' found in various gnostic texts.

However, Abraxas also has enigmatic roots :

The attempts to discover a derivation for the name, Greek, Hebrew, Coptic, or other, have not been entirely successful.

Source : Wikipedia