What does "under the weather" mean — and why is it funny?

informal

Meaning

Slightly unwell or ill.

Where it comes from

A nautical phrase: seasick sailors were sent below deck, literally under the weather rail, to recover out of the wind and spray.

Why it is funny

The gentle humor is that it blames your cold on the sky. Instead of saying you are sick, it pictures you trapped beneath a low, gloomy cloud, as if bad weather were personally sitting on you. It is a soft, almost poetic excuse, and that politeness is the charm.

Used in a sentence

"I'm feeling a bit under the weather, so I'll skip the gym today."