Although the word stock of British and American English are practically identical, there are items in the vocabulary of each version of English that are original to one of them. Such is the status of the verb ‘gobsmack’, defined in the OED as follows:

slang (originally and chiefly British).
Transitive. To amaze, astound.

As to frequency of use, it is the predicative “gobsmacked” that one encounters most frequently in contemporary British speech, defined as

“Flabbergasted, astounded; speechless or incoherent with amazement.”

Also in common use is the participial adjective “gobsmacking,” defined as

“That causes astonishment; astounding, breathtaking, staggering.”

The etymology of ‘gob’ as given in the OED is itself of special interest:

Origin: Probably a borrowing from Irish. Etymons: Irish gob, Gaelic gob.
Etymology: Probably < Irish gob and Scottish Gaelic gob beak, mouth (Early Irish gop muzzle, snout, beak) < a Celtic base of uncertain, probably expressive, origin.

Therefore, to “gobsmack” literally means to hit in the mouth––an act that would certainly shock!

We Americans would do well to incorporate these words into our usus. They are, after all, gobsmackingly useful.