A: How about going to a movie tonight?
B: Rain check. I don't feel like hanging out today. I'm feeling a bit under the weather.
Note: This idiom means to "feel ill or sick." Not surprisingly, the origin of the idiom "under the weather" can be traced back to maritime sources. Originally it meant to "feel seasick or adversely affected by bad weather." In the old days, sailors who weren't feeling well were often sent below the deck, so they could get out of the weather. The term we use today has been shortened. Originally, the idiom was "under the weather bow," which is the side of the ship where the bad weather is coming.