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1.
Completely dumbfounded, shocked. From the Irish word "gob" meaning "mouth".

We were utterly gobsmacked when we spotted John at a restaurant on Friday night, after having attended his funeral that very morning!
by Terri M. December 07, 2005
 
2.
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.
by Yogi!! April 18, 2005
 
3.
Irish (possibly only Dublin, I'm not sure) slang meaning to kiss someone without warning.
Random onlooker: OMG! He just gobsmacked her! That looked painful.
by Francie July 24, 2005
 
4.
to unexpectedly or suddenly hit someone or something in the mouth. Often done in anger or with the intent to injure. Almost always done with whatever is nearest at hand.
"I gobsmacked that bully with my biology book."
"Billy said he was not going to get off my new bike. I gobsmacked him into changing his mind with my book bag."
by R Lord February 08, 2008