What does "talk the hind legs off a donkey" mean — and why is it funny?

informal, British

Meaning

To talk endlessly; to be extremely, exhaustingly talkative.

Where it comes from

A British phrase from the 1800s, built on an older idea that a patient donkey would stand and listen until, absurdly, its own legs gave way.

Why it is funny

The humor is the cartoon cause and effect. The phrase claims a person can talk so long and so much that a sturdy animal's legs simply drop off from the sheer endurance of listening.

Used in a sentence

"Don't get her started on gardening — she'll talk the hind legs off a donkey."