Flabbergasted??

[insider]

I heard someone say that they were “flabbergasted” the other day and I realized what a strange word it is. How did it come to mean "very surprised?

[Kalhoun]

I have no idea, but I always laugh at the word.

[RealityChuck]

According to the OED:

[First mentioned in 1772 as a new piece of fashionable slang; possibly of dialectal origin; Moor 1823 records it as a Suffolk word, and Jamieson, Suppl. 1825, has flabrigast to gasconade, flabrigastit worn out with exertion, as used in Perthshire. The formation is unknown; it is plausibly conjectured that the word is an arbitrary invention suggested by FLABBY or FLAP and AGHAST.]

[RealityChuck]

According to the OED:

[First mentioned in 1772 as a new piece of fashionable slang; possibly of dialectal origin; Moor 1823 records it as a Suffolk word, and Jamieson, Suppl. 1825, has flabrigast to gasconade, flabrigastit worn out with exertion, as used in Perthshire. The formation is unknown; it is plausibly conjectured that the word is an arbitrary invention suggested by FLABBY or FLAP and AGHAST.]

So, ultimately, no one really knows; there are just educated guesses.

[Kallisti]

Origin? I must say I’m flummoxed. Most vexing indeed.

[Moo_the_Magic_Cow]

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.

http://www.word-detective.com/091801.html#flabbergast
It seems the flap/flabby + ghast is the most common explanation.

[Moo_the_Magic_Cow]

Sorry, this is the full explanation:

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.”

[Docklands]

It has the same meaning as gobsmacked.

[samclem]

Michael Quinion’s take on the word is at http://www.worldwidewords.org/weirdwords/ww-fla1.htm

He basicall agrees with Morris, but a few slightly different deductions.