What does "madder than a wet hen" mean — and why is it funny?

informal, American

Meaning

Extremely angry and irritated.

Where it comes from

An American expression. Farmers noticed that a hen, if dunked in water to cool it down or break its brooding, came back out furious.

Why it is funny

The humor is the very specific, very small drama behind it. To picture pure rage, the phrase hands you one soggy, indignant chicken — and somehow that flapping, dripping outrage is the perfect measure of a temper.

Used in a sentence

"When they cancelled her flight twice, she was madder than a wet hen."