Flabbergasted
Etymology
Of uncertain origin, first recorded in 1772 as a fashionable new slang word. It looks like a blend of "flabby" (or "flap") and "aghast," the sound itself suggesting something knocked slack with shock, though no source ever confirmed the recipe.
adjective · /FLAB-ur-gas-tid/
Overwhelmed with astonishment; utterly shocked.
What it names
Astonishment of the jaw-dropping kind — surprise so complete it leaves you momentarily speechless.
Example
She was flabbergasted to open the door and find the whole extended family standing in the rain with cake.
Related idea
To be flabbergasted is to be stunned by astonishment; to be flummoxed is to be baffled into confusion — one is shocked by what happened, the other simply can’t make sense of it.
See also