wait for the other shoe to drop

The idiom is a product of late 19th and early 20th-century urban architecture, specifically the tenement housing of New York City and other industrial centres. These apartments were constructed with identical layouts, placing bedrooms directly above one another. The floors offered little in the way of acoustic privacy, ensuring that the nightly routine of a neighbour upstairs was shared with the occupant below.

The experience became a matter of predictable timing. A resident would hear the first shoe hit the floor as their neighbour prepared for bed; the internal logic of the situation then dictated that a second sound must inevitably follow. One waited for the other shoe to drop simply because the alternative—a neighbour retiring for the night with one foot still shod—was a breach of the established order.