Who da thunk it?

Who da thunk it?
Who'd a thunk it?
Who'd 'a thunk it.

It's a slang way of saying "Who would have thought it/so?" or "I can't believe it".
Кто бы мог подумать!

It is used to express surprise that something unexpected has happened. Thus, in more standard English, someone might say something like one of these instead:

.....Who would have expected that to happen?
.....Would anyone have ever expected something like that to happen?
..... I can hardly believe that actually happened!

or more specifically:

.....This comes as a surprise to us all!

Examples:

.....An online survey of 1,250 adults and teens conducted by Opinion Research Corporation revealed that up to 67 percent of America’s young people say they actually want to spend more time with their parents. Who da thunk it?

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It's a deliberate use of a non-standard verb form thunk instead of thought. Because the verb "drink" goes drink, drank, drunk it's easy to think that "think" goes think, thank, thunk. That's where the thunk comes from.

Who'd = Who would
a = have
thunk = thought

Imperfect and downright bad grammar is used all the time, and often to great effect, in literature, music and film. Artistic license gives authors and screenwriters permission to bend grammar rules, especially when writing dialogue. My favorite quote from Kathryn Stockett's wonderful novel, The Help, is: "You is kind. You is smart. You is important." The moment would be far less powerful if Aibileen's grammar were correct.

There are also numerous idiomatic expressions that use bad grammar, but which are so well-established that we repeat them without fearing criticism. We collectively understand that it's bad grammar, and that makes it okay. For instance, we don't say If it isn't broken, don't fix it because the expression is If it ain't broke, don't fix it. The word 'ain't' is acceptable only because the saying is so ingrained in our culture.

The same goes for thunk, in my opinion. But outside of that particular wording, be careful about using thunk. (snarkygrammarguide.blogspot.ru)

[Who would have thought?]