Flabbergasted is to be astonished or utterly confounded. The OED informs us that this colloquialism, in use since the late 18th century, is a purely arbitrary combination of the words flabby and aghast. A magazine article in 1772 cited it as a new vogue word. At that time, however, flabby meant weak or spent rather than the meaning of unfit and overweight that it has acquired today. Thus, if a person was flabbergasted they would be feeling spent and aghast, which is but a short step to being astonished and utterly confounded. See also Aghast.