1
general::
Await a seemingly inevitable event, as in Now that she has a good enough job to leave her husband, we're just waiting for the other shoe to drop. This expression alludes to a person awakened by a neighbor who loudly dropped one shoe on the floor and is waiting for the second shoe to be dropped. [Early 1900s]
American Heritage Idioms
2
general::
Phrase(s): wait for the other shoe to drop
Fig. to wait for the inevitable next step or the final conclusion. • He just opened his mail and moaned. Now, I’m waiting for the other shoe to drop when he finds the subpoena.
McGrawhill's American Idioms And Phrasal Verbs