'flabbergast' originates from English, possibly a blend of 'flabby' and 'aghast', where 'flabby' meant 'soft and loose' and 'aghast' meant 'struck with terror'.

'flabbergast' appeared in the late 18th century as a colloquial term and eventually became the modern English word 'flabbergasted'.

Initially, it meant 'to overwhelm with shock or surprise', and this meaning has largely remained the same in modern usage.