What does "dead from the neck up" mean — and why is it funny?

informal, humorous

Meaning

Very stupid; completely lacking in sense.

Where it comes from

A British comic insult. The body is alive and working, but everything above the neck is declared, bluntly, to have stopped.

Why it is funny

The humor is the cheerful brutality of the diagnosis. The phrase calmly pronounces the head clinically dead while the rest of the person carries on as normal, blissfully unaware of the bad news.

Used in a sentence

"He tried to charge his phone by holding it up to the light — dead from the neck up."