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
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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
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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
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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
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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
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being totally speachless.unable to think of anything to say.
i was gobsmacked when i won the lotto
by mike briggs July 13, 2003
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UK Slang--def.

When someone is left speechless, dumbfounded, taciturn, non-plussed.
Regan: I think I'll be moving back to California.
Kennedy looked at her, gobsmacked.
Kennedy: Have you thought this through? You're my best friend, and yr leaving me?
by Babydoll75 March 22, 2019
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