MYSTERY SANDWICH, 1. the principle of mystery is, of course, this: the only way anybody gets stuck to anything is by a mystery sandwich. A person cannot be connected to his body, but he can have a mystery between him and his body which will connect him. You have to understand this thing about the mystery sandwich. It’s two pieces of bread, one of which represents the body, and one of which represents the thetan, and the two pieces of bread are pulled together by a mystery. They are kept together by a volition to know the mystery. (PAB 66) 2. a thetan stuck to anything is of course just a mystery sandwich. Thetan, mystery, object–mystery sandwich. (SH Spec 48, 6108C31)
Hubbard, L. R., (1975) Dianetics and Scientology Technical Dictionary. Los Angeles: Church of Scientology of California Publications Organization.