Green’s Dictionary of Slang

flabbergasted, amazed, speechless; thus gobsmack, a shock, gobsmacking, flabbergasting.

1936		Hull Dly Mail 1 May 10/6: Feller le’aked [looked] gob-smacked: Whats all wa dear? ee axes tother chap.
1956		J. Reynolds Woman of Bangkok (1959) 48: I’m so amazed that only the Malderbury dialect can express my condition: ‘I’m properly gob-smacked’.
1982		A. Bleasdale ‘George’s last Ride’ in Boys from the Blackstuff (1985) [TV script] 245: Are you really going to die, grandad? (A brief gob-smacked silence).
1998		R.G. Barrett Mud Crab Boogie (2013) [ebook] [H]alf gob-smacked and half-perving on all the trim young bodies and grouse big dicks.
1992		S. Armitage ‘Revision Exercise with Textbook Examples’ in Kid 72: But to us, gobstruck / behind screens and keyboards.
1998		Newall ‘Sir Gawayne and the Grene Knyght’ in Indep. Weekend Rev. 26 Dec. 1: Gawayne, gobbesmackede atte being setuppe.
1998		Mail & Guardian (Johannesburg) 11 Sept. 🌐 An English bobby who weyes only for the gobsmacking price of the new Polo.
1999		J. Cameron Brown Bread in Wengen [ebook] Put a bit of a gobsmack on Bernard though. Reckoned he might get a ticker job on the fucking spot.
2006		E. St Aubyn Mothers Milk 50: They’re absolutely gobsmacked – they’re loving parents.
2011		P. Tatchell on ‘Excess Baggage’ BBC Radio 4 [radio] Madeira - I was gobsmacked when I went there.
2011		Week 8 Jan. 25/2: A gobsmacking spectacle.
2023		(con. 1962) J. Ellroy Enchanters 329: She was gobsmacked. She did a broad double-take.
2024		J. Meades Empty Wigs (t/s) 831: [H]e’d never seen anyone so open-mouthed. Gaping [...] like the proverbial village idiot.