A drainpipe is a pipe that carries water away from where it's not wanted. You'll see drainpipes running down the sides of houses, catching rainwater from gutters and directing it safely to the ground so it doesn't damage the building's foundation. Under sinks and bathtubs, drainpipes carry used water away to the sewer system.

When rain pours onto your roof, it flows into gutters, then into vertical drainpipes that guide the water away from your house. Without drainpipes, water would pool around buildings, seep into basements, and cause serious damage.

In older British slang, people sometimes called extremely tight-fitting pants drainpipe jeans because they were so narrow they looked like pipes. But that meaning has mostly faded away, and today when someone mentions a drainpipe, they almost always mean the actual pipe carrying water where it needs to go.