[This is a fast-access FAQ excerpt.]
This phrase means "to await an event causally linked to one that
one has already observed". In the form "drop the other shoe",
meaning "say the next obvious thing" or "end the suspense", it dates
from the early 20th century. It derives from the following joke:
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?"
Markus Laker reports that The Goon Show (a 1950s BBC radio
comedy) made reference to this. The character Eccles was an idiot
and a bit of a freak.
"What's that noise?"
"Oh, that's just Eccles taking his boots off."