What does "happy as a clam" mean — and why is it funny?

informal

Meaning

Very happy and content.

Where it comes from

Short for the fuller American phrase 'happy as a clam at high tide' — at high water, clams are safe from the diggers who hunt them at low tide.

Why it is funny

On its own the phrase is delightfully baffling: a clam has no face, no smile, no visible sign of joy at all. The humor is that we cheerfully picture a thrilled mollusc while having no idea what a happy clam would even look like.

Used in a sentence

"Give him a book and a quiet corner and he's happy as a clam."