Etymology
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flabbergast (v.)
1772, flabbergasted, mentioned (with bored) in a magazine article that year as a new vogue word, of uncertain origin. Perhaps from some dialect (in 1823 flabbergast was noted as a Sussex word), perhaps ultimately an arbitrary formation alluding to flabby or flapper and aghast. "Like many other popular words expressing intensity of action, ... not separable into definite elements or traceable to a definite origin" [Century Dictionary]. Related: Flabbergasted; flabbergasting; flabbergastation.
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gobsmacked (adj.)

also gob-smacked, "flabbergasted, amazed, astounded," literally "smacked in the mouth," by 1936, U.K. slang, from gob (n.2) "mouth" + past participle of smack (v.).

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