1 definition by Tim Grim

Completely dumbfounded speechlessastounded. From the Irish/Scottish/Northern England word "gob" meaning "mouth".

I think it comes from the action of clapping your hand over your mouth when you are utterly surprised to the extent that you are unable to speak.
All the soccer fans were totally gobsmacked when Leicester City won the Championship.
by Tim Grim April 30, 2016