SURPRISE, ASTONISH, ASTOUND, AMAZE, FLABBERGAST mean to impress forcibly through unexpectedness.

SURPRISE stresses causing an effect through being unexpected but not necessarily unusual or novel.

e.g. surprised to find them at home

ASTONISH implies surprising so greatly as to seem incredible.

e.g. a discovery that astonished the world

ASTOUND stresses the shock of astonishment.

e.g. too astounded to respond

AMAZE suggests an effect of bewilderment.

e.g. amazed by the immense size of the place

FLABBERGAST may suggest thorough astonishment and bewilderment or dismay.

e.g. flabbergasted by his angry refusal

source: [SHIROBAKOの宮森あおいが電話で仰天するGIF画像｜無料GIF画像検索 GIFMAGAZINE 699647]

verb with object (usually as adjective flabbergasted) informal

surprise (someone) greatly; astonish:

｟くだけて｠ ; 〖通例be ～ed〗 «…に» びっくり仰天する «at, by» .

e.g. this news has left me totally flabbergasted.

late 18th century: of unknown origin.