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.
Gob
A gob of something is a pile or lump. Gobs of something is a lot of that amount, though this usage is mainly in North America.
A gob may also be someone’s mouth
To gob is to eject saliva from one’s mouth, or spit. This definition is mainly used in British English.
In days past an American sailor could be called a gob, but this is largely out of use.
Other Uses
Another version of this adjective is gobsmacking. Usually things are gobsmacking while people are gobsmacked.
The term’s origins are pretty literal. When people are shocked or in awe, they clap or smack their hands to their mouths or gobs.
Another related term is a gobstopper, or a jawbreaker in the United States. The candy is round and hard, usually meant to be sucked on instead of chewed, effectively stopping one’s gob from other tasks such as talking.
Some, mostly Americans, erroneously think that gobstopper must be capitalized because it was used as a candy name in Roald Dahl’s Charlie and the Chocolate Factory for the everlasting gobstopper. However, Dahl did not create this word and even if he did, it is not a trademarked candy, as it is fictional.
Examples
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]