What does "like nailing jelly to a wall" mean — and why is it funny?

informal, humorous

Meaning

Describes a task that is impossible because the thing keeps slipping away and will not hold still.

Where it comes from

A modern comic phrase. Jelly is wobbly, soft and slippery, so fixing it in place with a nail is doomed from the start.

Why it is funny

The humor is the gloriously hopeless image. You can picture the whole sorry scene — the hammer, the nail, the jelly sliding straight down the wall — and you know it will never, ever work.

Used in a sentence

"Getting a straight answer out of him is like nailing jelly to a wall."