Our friends at Word Origins have thses entertaining tid-bits:
"...Sometimes it seems that the best words in the English language are like a good stew: wonderful flavors, but often you can't quite figure out what's in there. "Flabbergasted" is a marvelously vivid word, but it doesn't look like any other word we know in English, so its origins aren't easy to figure out. By now I'm sure you've looked up "flabbergast" in your dictionary and found that it says something like "Of unknown origin." That's true, strictly speaking, but we can trace "flabbergasted" at least part of the way back and make an educated guess as to the rest of the ingredients.
"Flabbergasted," by the way, is far from being a new word. It's been around since the late 1700s in its current form. The second part of the word, "gast," is probably from the Middle English word "gasten," meaning "to terrify," which also gave us "aghast." "Gasten" itself comes from the Old English word "gast," or "spirit," which also gives us "ghastly" and "ghost." So there we have the "surprise" part of "flabbergast."
The "flabber" part is the puzzle. Most likely, it's related to "flabby," which itself is a variant of "flappy." (Yes, to say someone is "flabby" is to say that they "flap" when they move, which is enough to send anyone to the gym.) But "flap" can also mean excitement or a disturbance ("The flap over the Royal Family"), so this is where the guesswork comes in. "Flabbergasted" may have originally meant being so surprised that one "flabbed" -- trembled like Jell-O. Or it could have referred to the cause of the uproar -- the "flap" at which one was "aghast," or "flabbergasted..."