1772, mentioned (with bored) in a magazine article as a new vogue word, perhaps from some dialect (in 1823 it was noted as a Sussex word), likely an arbitrary formation from flabby or flapper and aghast.
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."
The British comedian Frankie Howerd used to say in mock astonishment: “I’m flabbergasted " never has my flabber been so gasted!”. That’s about as good an explanation for the origin of this word as you’re likely to get. It turns up first in print in 1772, in an article on new words in the Annual Register. The writer couples two fashionable terms: “Now we are flabbergasted and bored from morning to night”. (Bored " being wearied by something tedious " had appeared only a few years earlier.) Presumably some unsung genius had put together flabber and aghast to make one word.
The source of the first part is obscure. It might be linked to flabby, suggesting that somebody is so astonished that they shake like a jelly. It can’t be connected with flapper, in the sense of a person who fusses or panics, as some have suggested, as that sense only emerged at the end of the nineteenth century. But flabbergasted could have been an existing dialect word, as one early nineteenth-century writer claimed to have found it in Suffolk dialect and another " in the form flabrigast " in Perthshire. Further than this, nobody can go with any certainty.
It is believed that the word “flabbergasted” came into the English language as a slang expression. Then it was devised as a slang expression to express something like “overcome with surprise and bewilderment”. It is believed that the word is an amalgamation of the words flabby and aghast. The first half meant something like shaking in the form of or very much like jelly. The latter half of the word is believed to mean something like struck with amazement or horror.
In modern times the word is used as a part of normal speech, not just slang, and refers or means to cause someone to be overcome with sheer astonishment. “The behaviour of the children from his school left the headmaster flabbergasted” is an example of the word being used practically.