There’s a stomach flu going around, so I shouldn’t have been surprised when I heard someone in the supermarket announce I’m so nauseous. I mean, when you’re sick, it can get pretty bad for everyone around you. I don’t normally turn to look at every noise or loud conversation, but I couldn’t help it this time; I was curious.
When I saw the young woman, I was shocked that she would have referred to herself as nauseous; sick or not, she was actually quite attractive. Apparently, this was a simple case of mistaken grammar identity: She had mistaken nauseous for nauseated.
Here’s how I see it: Some people insist that nauseous really means to feel sick (I’m nauseous means I’m sick), while others insist that it means causing nausea in others (I’m nauseous means I’m so disgusting that I’m causing you to be sick). It seems that everyone agrees, however, that nauseated means to feel sick (I’m nauseated means I’m sick). That leaves us with two words that potentially mean I feel sick and one word that means I’m making others sick.
Actually, there’s an argument for using either nauseous or nauseated to mean I feel sick. Different editions of different dictionaries, for example, the American Heritage Dictionary, the Oxford English Dictionary, and Merriam-Webster’s, give different advice, usage explanations, and histories. Some say that nauseous in the sense of feeling sick is gaining in popularity, and others say that nauseous should be used only to mean causing nausea; there were so many other nuances that there doesn’t seem to be a straight answer. Meanwhile, the Online Etymological Dictionary says that in 1604, nauseous meant to be sick, and it didn’t come to be used to mean causing someone else to be sick until 1612.
While that all may be true, it doesn’t make sense to have one word mean two contradictory things (nauseous meaning I’m sick AND I’m making you sick); that would cause unending confusion between writer and reader (or speaker and listener). I’m not saying that I’m questioning the American Heritage sources, the Oxford sources, or the Merriam-Webster sources—well, not entirely—-I’m just saying that there is too much conflicting information out there to be sure about any single explanation.
My advice is to use nauseous to mean making someone else sick and nauseated to mean feeling sick yourself. If it makes you feel better (I know it makes me feel better)—and safe from confusion because the dictionaries are really of no substantial help—-then use nauseating instead of nauseous to mean making someone else sick (That’s a nauseating story) and nauseated to mean feeling sick (I’m nauseated from that story). That way, regardless of which dictionary you follow, there won’t be any confusion or embarrassment. This is also what most Usage Panels suggest.
Sherry
Sherry’s Grammar List and Paul’s book–Literally, the Best Language Book Ever
Everything Language and Grammar 
Hallelujah for an intelligent, cohesive, persuasive evaluation of this usage, which is one about which a friend of mine (also an English teacher) and I have just had rather a set-to. I fully agree with you; he seems to believe that we must simply acquiesce to the common herd’s usage, despite the logic. Thanks!!!
Reply from Sherry: You’ve hit on a very important point: Many people believe that we should, as you put it, acquiesce to the common herd’s usage despite the logic. This approach, in too many instances, seems to be reflected in modern dictionaries. The thinking seems to be that if it’s used often enough, even if it’s a true grammar error, then we should no longer call it an error but, rather, an alternative, standard meaning or usage. While language does, of course, evolve, there should be a reason and a logic for the evolution, which, for me, never includes standardizing a mistake.
Thanks for writing!