Snuck in

Hi,

What does this phrase "Who'da thunk it" mean?

Thanks!

It translates to:
"Who would have thought anything like THAT would have happened".

It's a lighthearted thing to say. The perfect tense of think is thought. This saying pretends that the past tense of think is thunk, just as the perfect tense of slink is slunk.
Similarly, the perfect tense of sneak is the regular form sneaked, but you can often hear (American) people say snuck. This usage seems to be growing. Is it regarded as correct in America?

My Associated Press Stylebook says "sneaked" is the proper past tense of "sneak" and cautions: "Don't use the colloquial 'snuck.'"