If something is described as "noisome", what characteristic does it have?

If something is described as “noisome,” it is foul-smelling. The adjective is most commonly used for an odor, place, or substance that is highly unpleasant or offensive to the sense of smell.

The word has a misleading appearance: noisome does not mean noisy. It comes from Middle English noysome, derived from noy, an older word related to annoyance, combined with the suffix -some. Its spelling may suggest sound, but its modern everyday association is with a bad smell.

Historical English also used noisome in a broader sense meaning harmful, unhealthy, or noxious. That older meaning still appears in literary or formal writing, but “foul-smelling” is the clearest answer in a vocabulary quiz, especially when the word describes a stench, drain, swamp, or decaying material.

Useful near-synonyms include fetid, malodorous, putrid, offensive, and rank. “Noisy” describes excessive sound; “noisome” describes something offensive, especially an odor.