Mothball is a small ball of strong-smelling chemicals that people place in closets and storage boxes to keep moths and other insects away from clothing. Moths lay eggs in wool, fur, and other natural fabrics, and when the eggs hatch, the larvae eat holes through the material. Mothballs release fumes that repel these pests.

The chemicals in mothballs have a distinctive, sharp odor that many people find unpleasant. If you've ever opened an old trunk in someone's attic and smelled something strange and chemical, you might have been smelling mothballs. Because of their strong smell, mothballs have become associated with old, unused things.

This leads to the second meaning: to mothball something means to store it away for a long time without using it. A navy might mothball old ships, keeping them preserved but inactive. A company might mothball a project, setting it aside until conditions improve. When something is mothballed, it's not thrown away or destroyed, but it's not in active use either. It's in a kind of suspended state, waiting.

If someone describes a building as smelling like mothballs, they usually mean it seems old-fashioned or hasn't been updated in decades. The word carries a sense of something preserved from the past, protected but frozen in time.