According to the book 'The Queen's English: An A to Z Guide to the English in Their Own Words', this wonderful term comes from joining two slang words 'gob' which is slang for mouth and smacked (which also relates to being kissed on the mouth). Together the word 'gobsmacked' means 'left speechless'.
The term 'gobsmacked', writes C.J. Moore, came to public fame on the lips of leading Conservative politician, Chris Patten, a member of the Thatcher administration. 'Double whammy' (double blow) and 'porkies' (= pork pies = lies) were other terms he used. Patten later "went to annoy the Chinese as last Governor of Hong Kong".
To purchase 'The Queen's English: An A to Z Guide to the English in Their Own Words' by C.J. Moore, click HERE.