Noisome means disgusting or harmful, especially because of a terrible smell. A noisome odor is so foul it makes you want to cover your nose and run away. The word comes from an old word meaning annoy, and noisome things are certainly annoying, but in a much stronger way.

You might encounter noisome fumes from a chemical spill, or describe a dumpster behind a restaurant on a hot summer day as noisome. A swamp filled with rotting vegetation could release noisome gases. The word suggests something actively unpleasant and potentially unhealthy: a smell you can't ignore.

Here's something surprising: despite how it sounds, noisome has nothing to do with noise. A loud classroom isn't noisome (though it might be noisy). A smelly, polluted factory could be both noisy and noisome, but those are two separate problems. If you see noisome in your reading, it's always about something offensive to your sense of smell or harmful to your health, never about sound.