The phrase 'Under the Weather' is used when you’re not feeling good.
Example of Use: “I don’t feel like hanging out today. I’m feeling a bit under the weather.”
Not surprisingly, the origin of the idiom 'under the weather' can be traced back to maritime sources. In the old days, sailors who weren’t feeling well were often sent below deck, so they could get out of the weather. The term we use today has been shortened though; originally, the idiom was 'under the weather bow' which is the side of ship from which the bad weather is coming.