The method for determining the identity of a target based on a process of elimination. The term is often applied for humorous effect meant to indicate that a solution can easily be found by simply eliminating the obvious non-solutions.
The concept is based on the animated TV show "Scooby Doo" in which teenage investigators solve mysteries. The typical episode format includes someone dressed as a monster terrorizing people. The "meddling kids" proceed to determine the identity of the person behind the mask, providing an opportunity for the audience to guess who it is as well.
As an over-simplification, you know the person behind the mask simply because he’s the only person in the episode who isn’t Fred, Daphne, Velma, Shaggy, or Scooby (the primary characters).
Brilliant, you just reinvented the Scooby Doo Algorithm to choose what to eat for breakfast. . . It's not that hard since you only have one box of cereal, half a pizza, and ten cases of beer in the house.