What is the meaning of ‘gobsmacked’?

This is a word which is mostly used in informal contexts in British English. It is a combination of two words: ‘gob’ and ‘smacked’. The word ‘gob’ is often used in informal contexts to refer to the mouth. The word ‘smacked’, as we know, means to hit someone rather hard. If someone were to hit you hard on the mouth, how would you react? Your automatic reaction would be to cup your hand over the mouth. You would render yourself speechless for a few seconds. The word ‘gobsmacked’ is used to mean ‘utterly astonished, astounded’; you are left totally speechless.

*When the CEO was arrested, we were, like everyone else, gobsmacked.