If you've ever heard someone say they're under the weather, rest assured this expression has nothing to do with hail, sleet, or snow. Instead, people say "under the weather" to express that they're feeling ill or unwell.
Historians believe this idiom comes from the sea. In the days before airplanes, people usually traveled by ship.
During storms, the seas would get rough, causing ships to rock back and forth. The rocking motion often caused passengers to become seasick.
Seasick passengers would head below deck to a lower point where the rocking was less noticeable. Passengers were thus forced under the deck by the weather…and the expression "under the weather" was born!