to be astounded: overwhelmed by an awe that mixes wonder and dread
The word names far more than surprise. It is the reaction of someone stopped in their tracks by something overwhelming, a wonder so strong it carries a shiver of fear. Ancient Greek used it for the stunned awe that grips people at the sudden appearance of a god, at a portent, or before overwhelming power, and that older picture of a person rooted to the spot stays attached to it.
That is why one English word rarely fits. The feeling holds wonder and dread together at the same time, delight and alarm mingled. Depending on the scene, translators reach for "amazed," "astonished," or even "trembling," and each choice keeps one side of the emotion while letting the other slip.
In the New Testament it appears only in Mark, and once in a later manuscript addition to Acts. In Mark it marks the moments when Jesus overwhelms the people around him: the crowd stunned by his authority in the synagogue, the disciples shaken by his hard words about wealth, the Twelve gripped by foreboding as he leads them toward Jerusalem. It sits at the heavy end of a family of Greek astonishment words, stronger and darker than the plain marvel of θαυμάζω or the mind knocked off balance in ἐκπλήσσω, and it has an even more intense cousin, ἐκθαμβέω, which Mark saves for its most extreme moments.