What does 'dead from the neck up' mean?

[Zishuli]

English Language Tests, Intermediate level

ESL/EFL Test #330 "English Slang Idioms (51)", question 6

I don't know why Jay doesn't fire Ted. That guy is dead from the ......... up. He can't even get the simplest tasks done.

(a) ground
(b) pick
(c) brain
(d) neck

English Language Tests, Intermediate level

ESL/EFL Test #330 "English Slang Idioms (51)", answer 6

I don't know why Jay doesn't fire Ted. That guy is dead from the neck up. He can't even get the simplest tasks done.

Correct answer: (d) neck

Your answer was: correct
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Hi,
May I know what does" be dead from the neck up" mean?
Thank you.

[Mister Micawber]

.
Stupid.
.
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Native English teacher at Mister Micawber's