Under the weather
Somewhat indisposed, not feeling well, dates from the early 19th century. Although the origin is not certain, it is reasonable to conjecture that the origin is nautical. Some sources suggest that when sailors were unwell, they were sent below i.e. under or out of the weather. Other sources maintain that the original expression was ‘under the weather bow’, the bow of the ship being the worst place to be because it was constantly buffeted by the weather. Being sent below decks if unwell i.e. under or out of the weather, seems more plausible.