To flabbergast is to shock, awe, bowl over. For something to flabbergast you, it ought to leave you sputtering in amazement.
The word flabbergast dates back to 1772, but its origin is unknown. Perhaps its creator had in mind both being aghast and the sputtering sound made by people who are so amazed, they can barely speak. To flabbergast is to shock, so some events that might flabbergast you include winning the lottery, hearing that your best friend and his family moved to Switzerland yesterday without telling you, or getting stuck in a snowstorm in April in Texas.