English loves compound words: 'washing machine' and all that sort of thing. But when you get a compound word, the two parts of the compound are usually stylistically very homogeneous, in other words, they are the same style: formal first part - formal second part, and so on. You don't usually get a compound word where the first part is a slang thing and the second part is a rather ordinary or formal thing - they don't usually mix - but gobsmacked is a perfect exception to that rule.
To be gobsmacked - it means to be astounded, flabbergasted, speechless with amazement. And what you've got is the perfectly ordinary word 'smacked' (to smack) and a Northern word 'gob'. Gob is the Northern word for mouth. I used to live in Liverpool for many years and you'd say to people "ah, shut yer gob", you know, and it simply means, mouth. So, to be 'gobsmacked' is to be struck dumb as if by a smack in the face.
Now, it's got a more general use these days than just in the North. I've heard it used throughout the South of England - I've heard it used abroad; it's now very widely used to be gobsmacked. Now, why? Because it became a very fashionable expression by people on television, not everybody on television, I'm thinking especially of people like sports personalities having a terrible day, something horrible happens: footballers in particular are always saying that they're gobsmacked at something happening.
Transcript (pdf - 45k)
Lesson plan - Teacher's notes, student worksheets with answers (pdf - 90k)
Audio - Professor David Crystal on "Gobsmacked" (mp3 - 880k)
|^^ Top of page|