To await an event that is expected to happen, due to being causally linked to another event that has already been observed. Gets its origin from the joke in the example.
A guest who checked into an inn one night was warned to be quiet because the guest in the room next to his was a light sleeper. As he undressed for bed, he dropped one shoe, which, sure enough, awakened the other guest. He managed to get the other shoe off in silence, and got into bed. An hour later, he heard a pounding on the wall and a shout: "When are you going to drop the other shoe?" Thus the term "waiting for the other shoe to drop."
by Bill Koch March 19, 2007