What does "happy as a pig in mud" mean — and why is it funny?

informal, humorous

Meaning

Completely happy and content.

Where it comes from

An old farmyard comparison. Pigs really do love to wallow in mud, which keeps them cool and free of pests.

Why it is funny

The humor is the cheerfully undignified picture of bliss. To describe perfect contentment, the phrase reaches not for anything graceful but for a pig, up to its ears in mud and delighted about it.

Used in a sentence

"Give him a fishing rod and a quiet river and he's happy as a pig in mud."