By now it's generally accepted that the magical phrase "Hocus pocus" is a corruption of the Latin liturgical phrase "Hoc est corpus meum" ("This is my body...") spoken by Catholic priests at the critical moment in the Mass.
Unfortunately, the magical phrase "Abracadabra" does not have a similarly well-established origin. Here is my hypothesis.
Through a mechanism of corruption like that which produced "Hocus pocus", "Abracadabra" has its origin in the Latin liturgical phrase "Ave verum cadaver" -- "Hail, the true cadaver", i.e. of the slain Christ.
(However, it is interesting to note that "Abracadabra" sounds more similar to the plural form, "Ave vera cadavera". What bodies this could have been referring to, I don't know...)
