Flabbergasted
verb
overcome with amazement
“This boggles the mind!”
Similar: flabbergast, boggle, bowl over
adjective
as if struck dumb with astonishment and surprise
“a circle of policement stood dumbfounded by her denial of having seen the accident”
Similar: dumbfounded, dumfounded, stupefied, thunderstruck, dumbstruck, dumbstricken