Feeling slightly unwell, especially after a night of indulgence. This phrase has a maritime source, though there are differing explanations of it.
During the days when ships were powered by sail, and now as well, the captain’s log documented everything that happened during the day. As sickness could spread rapidly on a ship, there were often times where the number of sailers that were ill exceeded the space provided in the log to record their names. When this happened, the excess names of the sick were recorded in the next column, which was reserved for the weather conditions of the day. Thus, it was not unusual for a sick sailor to be listed “under the weather.”
The second theory is this: In the old days, when a sailor or a passenger felt unwell, he was sent down below to help his recovery, under the deck and away from the weather.
The earliest recording found of the idiom is from the newspaper Jeffersonville Daily Evening News, 1835, and it says: “‘I own Jessica is somewhat under the weather to-day, figuratively and literally,’ said the gentleman, amusedly, giving a glance at the lady over in the corner.”
I much prefer the first explanation, to do with the ship’s log. Having spent much of eight months on board a freighter, I know that being below deck protects you from the weather, but it’s also the worse place to be if you’re feeling seasick. The best place is on deck, in fresh air, and with your gaze focused on the horizon rather than on the heaving waves.
Been there, done that. Have no desire to repeat the experience!