It is NOT often used by older people as Gary says. You could live for many years among older people and never hear it. Jessica has the best answer.

Its means "Who would have thought it?" This is a phrase often used by older people. Whilst "woulda'" is not an official English word, rightly or wrongly is perfectly acceptable in spoken (not written) English these days. "woulda'" is even considered as regional dialect when used in spoken English. "Thunk" although little used these days is in the dictionary as a past tense of "think". The phrase as a whole is used when you have been told something that you would not expect to be true. A piece of information that has surprised you.

What that means is a slured form of "Who would of thought that". it's really only used when you try to be funny, but really you use it with your friends. in everyday life you don't want to use it, you might sound stupid.

Ciao Mauro! Yes it's slang for 'who would have thought it' (not 'who would OF thought it' which is grammatically incorrect - apologies also to Jesica). I think the phrase might be what my father would call an 'Americanism' as it sounds like American slang (perhaps New York?) but I could be wrong so I am happy to be corrected! And Leanna you are not old ;-)

It all depends on your definition of old, LOL. My age group has used it but I didn't/don't (I'm 47). I think it is possibly a regional phrase that was then used on television shows, like most slang. It's basically "Who would have thought it/that?" as several people have responded.