First recorded around 1765–75.
A variant of flabagast, which may be a combination of flabb(y), "hanging loosely or limply" or flap, "a blow given with something broad and flat," and aghast, "struck with overwhelming shock."
Sign up for more Word of the Day!

EXAMPLES OF FLABBERGAST

It's National Poetry Month, and we’ve created a challenge that really is for everyone. Submit now!

Did you mean substitutivity?