[A humble request: If you can, please donate to help me carry on tracing word histories. Thank you.]

 

In allusion to being brain-dead, the colloquial phrase dead from the neck up, also dead from the chin up, means: completely stupid.

This phrase is of American-English origin—as mentioned, for example, in the following from Bystander comments on the World War—and on those who wage it wisely and otherwisely, published in The Bystander (London, England) of Wednesday 19^th June 1918 [page 491, column 1]:

The Official Expert
The Ministerial defence of the taking over of the Cippenham farm-land was not well received in the House, or outside. Destruction of crops, in these serious times, demands a much better justification than it received; and one feels sorry for Ministers whose only defence is that the official experts have decided that so-and-so is the only  possible course to take. […] Too many of these self-opinionated experts appear to be, to  use an expressive American colloquialism, “dead from the neck up.”

These are, in chronological order, the earliest occurrences of the phrase dead from the neck up, also dead from the chin up, that I have found:

1-: From the Kokomo Tribune (Kokomo, Indiana, USA) of Thursday 13^th October 1870 [page 3, column 3]:

Premium List.

Up to the time of going to press nothing is heard from the premium list. The lazy Secretary will probably get it ready by Christmas and will hand it to some obscure sheet without circulation. But we shall print it before spring unless the Secretary hides it away as he did the posters and offered premium lists of the fair.
It is so nice to have a secretary of a fair who is dead from his neck up. The work goes on so rapidly—quite as rapidly as it would if a mummy were in his place.

2-: From the column Notes By The Way, published in The Public Ledger (Memphis, Tennessee, USA) of Tuesday 12^th January 1892 [page 2, column 2]:

The “Little Tycoon” […] is billed as a “comic opera,” though the only feature approaching humor is a caricature of an English swell, who is dead from the chin up and preternaturally rigid of limb, and who is propelled across the stage by his valet like a baby’s perambulator.

3-: From The Chicago Daily Tribune (Chicago, Illinois, USA) of Sunday 1^st October 1899 [page 54, column 7]:

Epigram Is His Weapon.

This particular Judge was a man “Mont” Cochran did not like, but by some oversight he had forgotten to say so. That evening he got some political gossipers into the editorial rooms of the Gazette and entertained them with one of his fetching monologues until he brought the Judge in as a subject for discourse. Then he gave the others a chance to get into the conversation by asking:
“What do you think of Judge ——, anyway?”
“O, he’s all right,” they agreed.
“Yes, he’s all right. He’d go to Congress some day, too, if it wasn’t for one thing.”
“What is that, Mont?”
“He’s dead from his chin up.”
This criticism served not only to give notice to the Judge that “Mont” Cochran didn’t like him, but it had the effect of blotting a dangerous rival out of the latter’s horizon, for it was repeated by everybody in the district as a laugh provoker, and the exact force of one of those things in a political way never has been fully measured.

4-: From the Lewiston Evening Journal (Lewiston, Maine, USA) of Wednesday 28^th August 1901 [page 5, column 2]:

THE PENNANT HUNTERS.
Maine Makes Another Bid for the Bunting—We Beat Manchester.

George Curtis’s circus horse was the only real lively guest that the Lewiston base ball team entertained, on Tuesday, at the beautiful little park across the gully in [?-ville] ¹. The Manchester team was down on the bills ² with such old-time terrors as Butcher Reagan, John Smith, Sheets-in-the-Wind and others, but they were all dead from the neck up and the horse was the only real article of merit outside the Lewiston team.

¹ Apart from its ending (which is -ville), the place-name is illegible.
² I have not found out what the phrase to be down on the bills means.

5-: From the column On Second Thought, published in The Topeka Daily Capital (Topeka, Kansas, USA) of Friday 10^th September 1909 [page 4, column 4]:

“He is alive in his legs,” said Buck Kilby yesterday when he came in from an attempt to interview Ezra Spann, “but he has been dead from the neck up for twenty years.”

6-: From one of the unconnected paragraphs making up the humorous column Daffy-Dills, published in The Great Bend Daily Tribune (Great Bend, Kansas, USA) of Tuesday 25^th April 1911 [page 4, column 1]:

Rubb—I always see Sue around a lunch counter.
Dubb—What Sue?
Rubb—Soup! Ha! ha!
Dubb tries to tell it.
Dubb—Sue is thick, but she’s popular, huh?
Lubb—Sue who?
Dubb—Irish stew. Haw! Haw!
(Dubb, you’re dead from the neck up.)