mightn't

Someone asked me if there is a contracted form of might not, like wouldn't for would not and couldn't for could not. Since I've never seen or heard it used, then I said I didn't think so.

Meanwhile I started to read a book called Sleeping Murder written by Agatha Christie, which gave me the very example of mightn't. I showed it to two of my colleagues who are from the states, and interestingly they got amazed to see this, too. One of them said that mightn't sounds rather posh and would rather be British English. I also looked it up in a British corpus, which gave me no example of it.

Throughout the year I've realised that every word is, however trivial, worth looking at.