The colloquial phrase to talk through (the back of) one’s neck means: to use extravagant words or language not substantiated by fact; to talk nonsense.
This phrase seems to date from the 1890s. These are, in chronological order, the earliest occurrences that I have found:
1-: From The Sydney Morning Herald (Sydney, New South Wales, Australia) of Monday 19th September 1892, about a court case:
A statement having been made in Adelaide by Mr. Murphy, labour member for Balmain, to the effect that “he saw Mr. Johnson, P.M., after the arrests, when the magistrate said he would arrest every chairman of the defence committee as soon as appointed;” “He has not done so,” added Mr. Murphy, “so I suppose he was talking through his neck that time.”
2-: From a letter to the Editor, published in The Consett Guardian (Consett, Durham, England) of Friday 17th November 1893:
Sir,—In your issue of Nov. 3rd the “Old Crow” replies to mine of Oct. 27th, but he either misquotes or misunderstands me. I cannot be further from the mark in my illustration of the case than the Crow was in his illustration. If I said that the Local Veto Bill was only meant for England, and not North-West Durham constituency, and so the Local Veto Bill could not benefit Stanley Good Templars if it were passed, so that there was no need of A. Jones voting to support it, this “Bird” would say it was exactly the reverse or vice -versa, and that I was talking through my neck.
3-: From The Atlanta Constitution (Atlanta, Georgia, USA) of Monday 19th February 1894:
The Griffin Call, discussing the gubernatorial race, says:
“These gentlemen don’t don’t [sic] talk through the ‘back of their necks,’ neither do they assert things that they have not fully ascertained the reliability of as facts.”
4-: From The Age (Melbourne, Victoria, Australia) of Friday 28th October 1898:
At Prell’s-buildings, Collins-street, yesterday, there was a prolonged meeting of the Cardigan Proprietary Gold Mining Company No Liability, Ballarat. Shareholders were not in a happy frame of mind, the burden of their complaint being “no gold.” […]
Mr. Haig, speaking as an old miner, said the company’s claim was more like a rabbit warren than a gold mine. It was a disgrace. A drive south was what was wanted.
The Chairman: You are talking through your neck. (Holding up a plan of the mine): If you know so much about it, tell us where to drive.
Mr. Haig (indicating the spot): Drive there.
5-: Cf. below, quotation from The Amateur Cracksman, by Ernest William Hornung.
6-: From the account of the deliberations of the Footscray Council, published in The Age (Melbourne, Victoria, Australia) of Thursday 3rd May 1900:
Towards the end of the meeting, Cr. Toohey was endeavoring to induce the council to lay metal on a small portion of a main road in Yarraville, which had been cut up by the sewerage works.
Cr. Fraser: Toohey is talking though his neck.
Cr. Toohey: Mr. Mayor, are you going to allow this?
Cr. Fraser: I say again, Toohey is talking though his neck, because he cannot talk any other way. There is nothing offensive in that.
Cr. Toohey: I insist upon it being withdrawn.
Cr. Fraser still persisted in his statement.
The British author Ernest William Hornung (1866-1921) used the phrase in several of his stories. For example:
1-: In The Return Match, published in the collection of short stories The Amateur Cracksman (London: Methuen & Co., 1899):
I looked at Crawshay, anticipating trouble; and trouble brewed in his blank, fierce face, in the glitter of his startled eyes, in the sudden closing of his fists.
“And what’s to become o’ me?” he cried out with an oath.
“You wait here.”
“No, you don’t,” he roared, and at a bound had his back to the door. “You don’t get round me like that, you cuckoos!”
Raffles turned to me with a twitch of the shoulders.
“That’s the worst of these professors,” said he: “they never will use their heads. They see the pegs, and they mean to hit ’em; but that’s all they do see and mean, and they think we’re the same. No wonder we licked them last time!”
“Don’t talk through yer neck,” snarled the convict. “Talk out straight, curse you!”
“Right,” said Raffles. “I’ll talk as straight as you like.”
2-: In The Shadow of a Man, as published in The Age (Melbourne, Victoria, Australia) of Saturday 15th September 1900—in rural Australia, a pair of newlyweds are determined to make their improbable union work:
“We won’t meet our troubles half way,” cried the young man, with virile common sense.” “Come! We love each other; that’s good enough to go on with. And we’ve got the station to ourselves; didn’t I work it well? So don’t let’s talk through our necks!”
The bush slang made the girl smile.
3-: In In the Shadow of the Rope, as published in Clifton Society (Bristol, England) of Thursday 2nd January 1902:
“You both filed the secret for future use!”
“Don’t talk through your neck, mister,” said Abel, huffly [sic]. “What are you drivin’ at?”
“You kept this secret up your sleeve to play it for all it was worth in a country where it would be worth more than it was in the back-blocks? That’s all I mean.”
“Well, if I did, that’s my own affair.”
The British author Pelham Grenville Wodehouse (1881-1975), too, used the phrase in several of his stories. For example:
1-: In Full Moon (New York: Doubleday & Company, Inc., 1947):
“My dear fellow, one of the first lessons you have to learn, if you intend to preserve your sanity in Blandings Castle, is to pay no attention whatsoever to anything my brother Clarence says. He has been talking through the back of his neck for nearly sixty years.”
2-: In Pigs Have Wings (New York: Doubleday & Company, Inc., 1952):
“It seems to you a bizarre idea. Far-fetched. Potty. The picture you are forming in your mind of me is that of a man talking through the back of his neck.”
3-: In Ring for Jeeves (London: Herbert Jenkins, 1953):
“My dear old pursuer of pumas and what-have-you, you say you’re talking about bally bookies, but what you omit to add is that you’re talking through the back of your neck. Neat that, Jeeves”
“Yes, m’lord. Crisply put.”