Akademik

кто бы подумал!
КТО БЫ МОГ ПОДУМАТЬ!; КТО БЫ ПОДУМАЛ! both coll
[sent; these forms only; used as Interj or indep. clause foll. by a что-clause; fixed WO]
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it is hard to believe (that the situation has turned out this way), no one could ever have predicted (that things would turn out this way):
- who would (ever) have thought?;
- who could have imagined?;
- it's hard to believe!;
- I never would have guessed!;
- (just) imagine!;
- [used as Interj only] what do you know!;
- imagine that!;
- incredible!
     ♦ [Сарафанов:] Нина! У меня никакого сомнения! Он твой брат!.. Боже мой... Ну кто бы мог подумать? (Вампилов 4). [S.:] Nina! I don't have any doubts at all! He's your brother!...My God. ...Well, who would ever have thought? (4b).
     ♦ ...Кто бы мог подумать, что Варвара не одна дома? А она таки действительно была не одна - с Анисьей Лобановой (Абрамов 1). Who could have imagined that Varvara would not be alone in the house? But in fact, she was not alone; she was with Anisya Lobanova (1a).

Большой русско-английский фразеологический словарь. — М.: ACT-ПРЕСС КНИГА. . 2004.