Under the Weather

The leading nautical theory holds that sailors who fell ill were sent below deck, away from the weather side of the ship where storms and rough conditions hit hardest, to recover out of the elements, literally under the weather. It's a widely repeated explanation in maritime phrase glossaries and fits the phrase neatly, but not every etymologist is convinced it's the whole story rather than a plausible tale attached after the fact to a simpler, more general use of "under" to mean oppressed or affected by something.