Gobsmacked is a British slang term meaning to be extremely surprised, like the gentleman in the picture. This informal slang term is derived from the words "gob" and "smack."
A gob is also an informal British slang word from Irish or Scottish Gaelic meaning "mouth." If you smack someone, you hit them. The image is almost exactly like the French expression "une claque dans la gueule."
Another common idiomatic expression with the same meaning employed in both British and American English is flabbergasted. The origins of this word are less certain. It has been around since the 1700's.
The first part of flabbergasted probaly comes from the word "flabby" or "flap" which means shaking uncontrollably like a bowl of jelly. If you say someone is getting flabby, we are saying that they are getting a little fat around the waist and their stomach is no longer firm.
The second half of the word probably comes from an obsolete verb "gast" from Middle English which means "to scare" or "to frighten. If we go back farther in English language history, this word also has its roots in the Old English word "gǣstan".
There is another modern English word that finds its roots in this Old English word. Can you guess what it is? If you saw one, you would be flabbergasted and gobsmacked!! The answer is a ghost (un fantôme)!
Here are a few example sentences using today’s vocabulary:
He was gobsmacked at how much weight he had gained.
He was flabbergasted at how much weight he had gained.