present participle of astound
"The news that he won the lottery by picking all six numbers was astounding to everyone at the office."
bewildering or striking dumb with wonder
"The sight of a dolphin leaping out of the water was so astounding that I forgot to breathe for a moment."
so surprisingly impressive as to stun or overwhelm
"such an enormous response was astonishing"
"an astounding achievement"
"the amount of money required was staggering"
"suffered a staggering defeat"
"the figure inside the boucle dress was stupefying"
That astounds or astound.
"The sudden announcement of his resignation was absolutely astounding to everyone in the room."