Hocus pocus is a corruption of the catholic text “Hoc est enim corpus meums”, said by catholic priests during transubstantiation sacrament ceremony and mean “this is my body”
Transubstantiation is the change of bread & wine, offered during the Mass, into the body and blood of Jesus Christ. It was perceived as a strong spell by church-going men and hence imitated by magicians.
However, some researchers assert it started by protestant magicians who rejected catholicism and wanted to make a mockery of the catholic transubstantiation ritual.