Gobsmacked, a British colloquialism, means (1) surprised, (2) dumbfounded, or (3) awestruck. In parts of Britain, gob is slang for mouth, and to be gobsmacked (one word) is to be figuratively smacked in the mouth—that is, struck dumb by something. Although gobsmacked takes the form of a participial adjective, there is no commonly used corresponding verb.
Gobsmacked is most common in British and Australian speech and writing. It appears occasionally in Canadian and U.S. publications, but it gives the impression that the writer is either British or affecting a British voice.
I remember seeing South Pacific for the first time on the screen and being gobsmacked. [Independent]
But the baker was left gobsmacked when Tendring District Council bosses refused the request, saying it would be too expensive to retrieve footage. [Daily Mail]
They were pleasantly gobsmacked by the transcendental naughtiness of the Carry On series, which loosed a species of gag that would have made Doris Day jump on a chair and scream. [Guardian]
Trainer Andrew Balding had been gobsmacked at Royal Ascot when Charles The Great failed to fire and finished tenth. [Mirror]