What does "a fish out of water" mean — and why is it funny?

informal

Meaning

A person who feels uncomfortable or out of place in an unfamiliar situation.

Where it comes from

A very old image, in use since the 1300s, of the obvious distress of a fish removed from the one place it can live.

Why it is funny

The gentle humor is the helpless flapping the phrase calls to mind. It captures social awkwardness perfectly — gasping, floundering, and visibly in the wrong element.

Used in a sentence

"At the formal dinner, in my old jeans, I was a fish out of water."