Under The Weather
To be ill.
I’d love to help you move all your furniture next weekend, but I expect to be feeling a bit under the weather.
Passengers aboard ships become seasick most frequently during times of rough seas and bad weather.
Seasickness is caused by the constant rocking motion of the ship. Sick passengers go below deck, which provides shelter from the weather, but just as importantly the sway is not as great below deck, low on the ship.
On a ship the greatest swaying action is on deck, and the most stable point is down near the keel. Hence seasick passengers tend to feel better below deck.
Some illnesses like rheumatism and arthritis act during time of poor weather. Sufferers from those ailments are literally under the influence of the weather.