What does "like watching paint dry" mean — and why is it funny?

informal

Meaning

Extremely boring; dull beyond endurance.

Where it comes from

A modern phrase. Watching paint dry is the very model of an activity in which, visibly, nothing happens.

Why it is funny

The humor is the deadpan extremity of the comparison. To capture pure boredom, the phrase picks the single most pointless way a person could spend an hour and dares you to picture it.

Used in a sentence

"The three-hour budget meeting was like watching paint dry."