The adjective gobsmacked means flabbergasted, astounded.

Apparently in reference to the shock effect of being struck in the mouth, this adjective is from:
– the noun gob, of Irish-Gaelic and Scottish-Gaelic origin, denoting the mouth;
– the adjective smacked, meaning struck, slapped.

The texts containing the earliest occurrences of gobsmacked that I have found seem to indicate that this adjective originated in Lincolnshire and Yorkshire, in north-eastern England. These early occurrences are as follows, in chronological order—the second and third are from novels by Hiram P. Bailey [cf. footnote], serialised in The Yorkshire Evening Post (Leeds, Yorkshire, England):

1-: From the column Thing we want to know, published in the Skegness Standard (Skegness, Lincolnshire, England) of Wednesday 25^th March 1925:

What the “deah boy” and the “comrides” were thinking about to get into the unhitched coach on the 8.38 train the other night?
And if they didn’t feel a bit ‘gobsmacked’ when the discovery was made that the rest of the train had gone without them?
And whether the ticket-collector has stopped laughing yet?

2-: From A Tale of the West. No. 2.—Impersonating Girl to Deceive Admirer, published in The Yorkshire Evening Post of Saturday 31^st July 1926:

“Well, what can I do for you, Jim?” I asked. “What’s the layout?”
“Jest this: I thought yew might dress up and pertend to be the young lady. Now don’t look scared. You might jest see Lido (the old negress help at the ranch-house) and persuade her to lit yew ’ev the loan of some of the young lady’s clothes. Yew ere thin. They’ll sure fit yew. Then yew might trot out on moonlight nights and entice Methody towards yew. Keep it up every night till we go North.”
“Great Scot, Jim. . . . (I felt gobsmacked.) I might try it for one night all right, but for nights on end—why, man, it’s impossible. I ain’t a miracle. . . . I ain’t the young British cove Henry Irving jest discovered.”

3-: From Rio Bound with Christine. No. 18.—The Mystery Woman, published in The Yorkshire Evening Post of Saturday 5^th February 1927:

“Ada Tuan, mem nangiss datang Tuan—Baniah. Baniah. (Yes, sir. There is a fine lady aboard. She cries very much.) And Wahpering gave a representation of his meaning. “Yes, Tuan, aladee—ees—ees here,” he attempted in English, using rather loud but distinct tones. “She cry, cry, cry, muchee cry.”
Chips, although silently working upon the other side the partition, had caught the full trend of the colloquy, and particularly had he caught these last words of the Malay. Abruptly he stopped mixing and kneading his white lead and tallow. If his blue, watery eyes ever could become dry they became dry upon this occasion. The ship’s carpenter gasped as he tilted his cap and fingered his hair in astonishment.
“Woman aboard. Woman, and ‘she cry; cry; cry’”; he breathed in the Steward’s accents.
“I’m bet. Gobsmacked.” And down he sat upon the top of a keg to think out what appeared to him a most consequential and extraordinary matter.

4-: From From a Yorkshire Clod-Hopper to John Hodge, published in The Bee-keepers’ Record: A Monthly Journal Devoted to Practical Bee-keeping (London: Simpkin, Marshall, Hamilton, Kent & Co.) of December 1935—as quoted by Hugo in Antedating of gobsmacked, on Friday 8^th June 2018:

That wad mack im about yam sick, so ower Ilkla’ moor bar-tat wish un best of luck at Ilkley Station, and when he landed back Martha wad be fare gob smacked at the yarns he wad tell ’er about Yorkshire clod-hoppers.

5-: From The Daily Mail (Hull, Yorkshire, England) of Friday 1^st May 1936:

GANNIN’ FER THE’ BE’AKS
Shepherd, Wagoner and Third Lad apply for Books under Farm Workers’ Unemployment Insurance

An imaginary scene (based on fact) at the village school where three farm workers have come to register themselves under the new unemployment scheme.
Temporary offices have been opened in some of the villages for farm workers to register, but the men are very slow and diffident about coming forward. They are shy, and treat the matter in the same way they would a visit to the dentists. Although it is understood there are 400 to register at Beeford, only 30 had appeared during the first week, and at Weaverthorpe only 69 out of 200.
Ship (shepherd), Wag (wagoner), and Thoddy (third-lad) get insured.
[…]
When feller ad axed arl questions ee could think on, ee ’ands pen ti Ship an ses: “Ere sign yer ne’ame there.”
Ship taks pen an puts a cross wer’ar man tells im.
“That we’ant doo,” ses man.
“I’ll ety!” ses Ship, “thoo sees ah carnt rite!”
Feller le’aked gob-smacked: “What sall we de’ar?” ee axes tother chap.
“Thoo steady ’is ’and fer im,” tother chap ses.
“That’s ne’ar good!” Ship ses, “ah de’ant knoa oo ti spell it!”

6-: From the following letter to the Editor, by ‘Zedekiah Tuttlethorn’, published in the Skegness Standard (Skegness, Lincolnshire, England) of Wednesday 10^th November 1937:

Deer Mester Standerd.—[…] One mornin’ […], the pooastman lands hissen at our plaace wi’ fower letters. “They’re all for ya, Zed,” he says.
“Fer me?” Ah says, scratchin’ me heead. “Why dang me, Ah’ve niver hed fower letters on one daay i’ me life afore.”
[…]
Well, mester, Ah were fair gobsmacked at them letters an’ tonned ’em ower ageean an’ ageean.

Note: The information that I have collected indicates that Hiram P. Bailey (died 1949, aged 74) was a civil engineer, novelist, lecturer and broadcaster who lived in Hull, Yorkshire. A Fellow of the Royal Geographical Society, Hiram P. Bailey was the author of a novel which first appeared in serial form in The Yorkshire Evening Post, from Saturday 17^th May to Saturday 30^th August 1924, under the title Shanghaied in the ’Nineties, and was first published in book form by Heath Cranton Limited, London, in 1925, under the title Shanghaied out of ’Frisco in the ’Nineties. In the Preface to the 1925 book, dated September 1925, the author explains that he “is not a sailor, but merely a civil engineer Shanghaied as one”, and that he “but depicts the life in the ship in which he was shanghaied—a normal American ship in many ways, yet exceptional in some”.