We all know how magicians saw people in half, but why do they say "abracadabra"? Many have wondered, and while theories abound, none definitively solve the mystery.
According to World Wide Words, the term first appeared during the second century in the Latin medical poem "De medicina praecepta." Apparently the poem's author, a physician named Quintus Serenus Sammonicus, believed the word could heal the sick when inscribed on an amulet and spoken in a particular way. Start with "abracadabra," then say "abracadabr," then "abracadab," and so on. Sort of like the kiddie ditty "B-I-N-G-O"--but in reverse.
As for how the word went from medical to magical, The ES Press believes abracadabra may have come from the Hebrew phrase "abreg ad habra," which means "strike dead with thy lightning."
Another theory argues the phrase comes from the Aramaic phrase "avrah kedabra," which translates to "I will create as I speak." Given magicians' flair for the dramatic, that makes sense to us.
Source: http://ask.yahoo.com/20060720.html