What does "cloud cuckoo land" mean — and why is it funny?

informal

Meaning

An unrealistic, over-optimistic state of mind; a fantasy world out of touch with reality.

Where it comes from

It comes from an ancient Greek comedy by Aristophanes, in which the birds build a city in the sky between the clouds.

Why it is funny

The humor is the giddy nonsense of the place name itself. To say someone is being unrealistic, the phrase posts them off to a fluffy imaginary town staffed entirely by cuckoos.

Used in a sentence

"If he thinks he'll finish the whole project tonight, he's living in cloud cuckoo land."