Moldy means covered with or containing mold, which is a type of fuzzy fungus that grows on food, wood, fabric, or other surfaces when they stay damp for too long. When bread turns moldy, you'll see spots of green, white, or black fuzz spreading across it. Cheese left too long in the refrigerator might develop moldy patches. A wet towel forgotten in a gym bag can turn moldy and smell musty.

Mold grows from tiny spores floating in the air that land on moist surfaces and multiply rapidly. While some molds are harmless or even useful (like the mold that makes blue cheese taste distinctive), many moldy foods should be thrown away because they can make you sick.

People use moldy figuratively to describe ideas or jokes that are stale and outdated, like calling something a moldy old tradition that nobody enjoys anymore. In this sense, moldy means something has been around too long without staying fresh or relevant, just like actual mold grows on things that have been sitting around too long in the wrong conditions.