very surprised or shocked
Origin and usage
The adjective flabbergasted was first used in the late 18th century. Early uses were mainly of the verb ‘to flabbergast’ but these days it is generally used adjectivally. The origin of flabbergasted is uncertain; it may come from a dialect word used in Suffolk or Perthshire, or it may have been created from the words ‘flabby’ and ‘aghast’.
The English language is very rich in words that mean ‘very surprised or shocked’, of which flabbergasted is one of the most colourful. Other colourful terms include ‘dumbstruck’, ‘nonplussed’ and ‘poleaxed’.
Perhaps because flabbergasted is such an odd and unusual word, people like to play around with it. So instead of saying they are flabbergasted by something, they say things like ‘my flabber is well and truly gasted‘.
“I’m flabbergasted. My gast has never been so flabbered.”
(Frankie Howerd in Carry on up the Jungle)
astounded, speechless, thunderstruck