Something noxious is harmful and could be even fatal — whether it is colorless, odorless, and invisible like carbon monoxide, or dark, strong-smelling, and obvious, like black smoke from a fire. People are called noxious too when they hurt others with cruelty or mental anguish.

English and Latin roots for noxious are related to the meanings of "harm," "damage," and "injury." While obnoxious might come to mind when seeing the word noxious, obnoxious is a much less damaging adjective to describe someone annoying. Most uses for noxious involve real physical or emotional damage unless the word is used in exaggeration. Examples range from "the noxious gas fumes made the dog pass out" to "his cologne was so noxious I had to exit the elevator and take the stairs."

Definitions of noxious

adj injurious to physical or mental health

noxious chemical wastes”
noxious ideas”
causing or capable of causing harm
baneful, deadly, pernicious, pestilent
exceedingly harmful
corrupting, degrading
harmful to the mind or morals
vesicant, vesicatory
causing blisters
of or relating to or caused by a toxin or poison
detrimental to physical or moral well-being
not injurious to physical or mental health
not causing or capable of causing harm
having no adverse effect
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