Proper usage of the word 'thunk'

Thunk occurs as the past tense and past participle of think in some regional dialects and is occasionally used in a jokey kind of way. In ‘Finnegan’s Wake’, Joyce wrote 'I then tuk my taken~place lying down, I thunk I told you’ and in ‘Ulysses' he used it as a noun: ‘Have a good old thunk’. It has, however, never been used in Standard English and its use would not make for effective communication in formal writing.

Imagine someone tells you something that you knew already and assumed was obvious to everyone else. You can convey your reaction by using the rhetorical question: "Who would have thunk it?"

There are numerous uses of the expression on the Guardian website. Two examples:

• Newsflash, films are made for profit. Who would have thunk it?

• Alcohol plus football equals fighting, apparently. Who'd have thunk it?

I would recommend against using "thunk" in any other circumstances.

As stated in the previous answers, thunk is informal. However, thunk as a noun is a legitimate computer-science term, derived from the the informal term's meaning of "thought of".