What does "as exciting as watching grass grow" mean — and why is it funny?

informal, humorous

Meaning

Extremely boring; utterly dull.

Where it comes from

A modern comparison. Grass grows far too slowly for the eye to see, making it the very model of an event in which nothing happens.

Why it is funny

The humor is the deadpan extremity of the picture. To convey total boredom, the phrase asks you to imagine sitting and staring at a lawn, waiting, hopefully, for it to do something.

Used in a sentence

"The two-hour safety briefing was as exciting as watching grass grow."