If you've just seen a ghost, or anything else that's terribly scary, then you're aghast: you're shocked, freaked out, and terrified. That word "aghast" is, we think, about seven hundred years old, and it may have led to the creation of the much newer, much goofier word, "flabbergast."
To flabbergast someone is to shock them or confuse them so badly that they have no idea what to do or what to say.
"Flabbergast" is a fun, silly, slangy word, and it's kind of a new word: someone probably invented it around the year 1772.
Let's see an example! In The Fellowship of the Ring, Bilbo flabbergasts his friends at a party by slipping on a magic ring and completely disappearing from sight. His sudden disappearance is flabbergasting. His friends are flabbergasted. And you know Bilbo totally did that on purpose: he's amused by his friends' flabbergastation.