Thunk is used as a pseudo-archaic past participle of think (by analogy with drink/drunk.) http://www.macmillandictionaryblog.com/whod-a-thunk-it The formal definition is here, https://dictionary.cambridge.org/dictionary/english/who-would-ve-thunk-it

Hey Tiger. You almost got it right. You have a really tricky sentence there. "Whoulda" is a contraction of "who would have." "Thunk," in this context, is really informal way to say "thought." So the sentence could also read "Who would have thought..." It's a phrase I might use in a Facebook post or a text to a friend. I wouldn't use it, however, in a business memo or a professional document.

You are correct. Trying to sound uneducated and with a "rural" accent, probably. Could be sarcasm.

It's basically short / slang for "who would have thought"