Where does the word “flabbergast” come from and What does flabbergast mean?

Someone whose identity will never be known dreamed up the word flabbergast about the year 1770.

At least it was reported as a new word in 1772, “Now we are flabbergasted and bored (another new word) from morning to night.”

Possibly the inventor coined it by joining forcibly the two words flabby and aghast, but he left no notes to furnish a definite clue.