“Mephitic” is an adjective that most often applies to a foul-smelling gas or vapor. “Noxious” is also an adjective, but it’s used within the definition of “mephitic.” It means “harmful or unpleasant,” and it could describe a substance, or even a person. These adjectives are all about the shades of meaning. That bothersome person who takes credit for your work could be described as noxious, but if she also wears way too much perfume, she’s mephitic.