Under the weather

That being my current state, my inner geek wondered where that came from. Aren't we always "under" the weather? Since the opposite would be "over" the weather (and I'm a bit delirious) I thought of three definitions.

Over the weather - Literally. as in a plane.
Over the weather - as in "the boss of". I think the Gorbical has that job.
Over the weather - as in sick of, "I am so over this weather!"

As with most idioms, this one is hard to track down. The best sounding answer I found here.

Moreover, the deck they were under was likely "the weather deck", meaning the most exposed deck on the ship, usually the foredeck (over the seamen's quarters at the front end of the ship) or the quarterdeck
(where the helm was located, high for a good view). Either way, if you needed to be sick, you wouldn't want to "toss your cookies" where the wind could toss them back in your face, nor would anyone with you want you to do so. You'd be firmly told to "under the weather" to be sick.

I'll leave you with that lovely mental image.