« I’m feeling a bit under the weather today, so I don’t think I’ll make it to your party. »
« Josh can’t come to school today because he’s a little under the weather, I’d like him to rest. »
If you’re feeling « under the weather » it means that you’re not at your best. Use it within a sentence when you’d like to tell someone that you’re not feeling well. It generally means you have a cold, and isn’t used for more serious illnesses.