What does "a damp squib" mean — and why is it funny?

informal, British

Meaning

Something that is a disappointing failure, especially after being expected to be exciting.

Where it comes from

A British phrase. A 'squib' is a small firework, and a damp one simply fizzles instead of banging.

Why it is funny

The humor is the soggy little anticlimax. All the build-up and expectation collapse into the picture of a firework that gives a feeble hiss and does absolutely nothing.

Used in a sentence

"After months of hype, the big launch was a damp squib."