A foghorn is a loud warning device that ships and lighthouses use to alert other vessels when thick fog makes it hard to see. The sound is deep, booming, and travels far across water, cutting through the fog to warn ships they're getting close to dangerous rocks, harbors, or other boats.

Before modern navigation technology like radar and GPS, foghorns saved countless lives. When dense fog rolled in and captains couldn't see more than a few feet ahead, they would listen carefully for the distinctive bellow of a foghorn to guide them safely. Different lighthouses had their own unique foghorn patterns, like a signature sound, so experienced sailors could identify which lighthouse they were near just by listening.

Traditional foghorns worked by forcing compressed air through a large horn, creating that unmistakable low, mournful sound. Some old lighthouses still use them today, though many have switched to electronic versions.

People also use foghorn to describe someone with an extremely loud, booming voice. If your uncle has a voice that carries across an entire restaurant, someone might joke that he sounds like a foghorn.