Gobsmacked
Etymology
British and originally Northern English, from "gob," a slang word for the mouth (likely from Irish or Scots Gaelic), plus "smacked." The image is doubly literal: either being struck in the mouth by shock, or clapping a hand over one’s open mouth in disbelief. It stayed regional for decades before spreading across British English in the late twentieth century.
adjective · /GOB-smakt/
Utterly astonished; struck speechless with surprise.
What it names
A vivid word for total, jaw-dropping astonishment — stronger and more physical than "surprised" or even "amazed."
Example
When they announced her name as the winner, she just sat there gobsmacked.
Related idea
To be flabbergasted is to be overwhelmed with astonishment; gobsmacked adds that specific image of a shock landing right on the mouth.