Green’s Dictionary of Slang

gobsmacked adj.

also gobstruck
[orig. northern dial.; lit. SE smacked in the gob n.1 (1)]

flabbergasted, amazed, speechless; thus gobsmacking, flabbergasting.

[UK]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’.
[UK]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).
[UK]S. Armitage ‘Revision Exercise with Textbook Examples’ in Kid 72: But to us, gobstruck / behind screens and keyboards.
[UK] Newall ‘Sir Gawayne and the Grene Knyght’ in Indep. Weekend Rev. 26 Dec. 1: Gawayne, gobbesmackede atte being setuppe.
[SA]Mail & Guardian (Johannesburg) 11 Sept. [Internet] An English bobby who weyes only for the gobsmacking price of the new Polo.
[UK]E. St Aubyn Mothers Milk 50: They’re absolutely gobsmacked – they’re loving parents.
[UK]P. Tatchell on ‘Excess Baggage’ BBC Radio 4 [radio] Madeira - I was gobsmacked when I went there.
[UK]Week 8 Jan. 25/2: A gobsmacking spectacle.