What you may think it means: to feel sick
What it actually means: to cause nausea
When you eat too much ice cream and declare to your mom or the nearest adult, “I feel nauseous,” what you’re actually saying is that you are causing people around you to feel sick. Thanks, jerk. (For the record, “I’m nauseated” is the way to go.)
People misuse nauseous all the time. I don’t because I’m awesome. Actually I don’t because I think it was in some movie? I can’t remember which but one character explains the proper usage to another. Thank god for movies or we’d never learn anything!