verb (used with object)
- to overcome with surprise and bewilderment; astound.
- informal overcome with astonishment; amazed; astounded
- informal to overcome with astonishment; amaze utterly; astound
1772, mentioned (with bored) in a magazine article as a new vogue word, perhaps from some dialect (in 1823 flabbergast was noted as a Sussex word), likely an arbitrary formation from flabby or flapper and aghast.