We hope you remember that idioms can't be understood literally. So, when somebody says that he is or feels UNDER THE WEATHER, it means that he doesn't feel good, he is ill or he feels sick.
You can ask, " So what does it have to do with the weather then?" You will have to read about the origin of this idiom to understand it better.
It all started back in the days when ships were powered by sail. The ship captain had to log documented everything that happened during the day. One of the columns in the log was for the names of the sailors who were sick that day. The second column was for the weather conditions. Sailors spent a lot of time together and did not have doctors or good medicine there, so the sickness spread very fast. As a result, the names of the sailors who were ill could not fit in the first column and the captain had to write the rest of the names in the second column of the log. And as you remember, it was THE WEATHER column. So, those sailors were UNDER THE WEATHER.
We hope you are not feeling under the weather today!