To be completely dumbfounded or shocked. That I know of, its origins are Irish.
I have know idea what a "gob" is or why it is common practice to smack somebody with one when they are shocked.
It seems like a rude thing to do, though.
"Well, I'll be gobsmacked!" exclaimed the taxi driver, as he watched two sworn enemies hug.
18 Nisan 2005, Pazartesi