The contraction of must not is musn’t!

And not mustn’t, even though we see it more and more often nowadays.

Even Oxford says that the right form is mustn’t (it doesn’t have musn’t) but still... I beg to differ.

Yeah, I know that is not becomes isn’t and was not becomes wasn’t. The two words are put together and the o is replaced by an apostrophe. But what about can’t, won’t and shan’t? They’re cannot, will not and shall not, and they do not follow that rule! And we all know that there are lots of exceptions in English, so musn’t is simply one of them!

The problem is that we find mustn’t very often. Way too often, in my opinion. (the same way that appears too often instead of who)

So here’s my theory: musn’t is used less often than can’t, and the misspelling doesn’t change its pronunciation. (unlike shan't and won't) So people misspell it, again and again, until it becomes the norm. And what about Oxford? I’d say that it got tired of correcting people, and decided to adopt the if you can’t beat them, join them attitude. In that case, shame on Oxford!

For the occasion, here's a quote that I absolutely agree with:
Ce n’est pas parce qu’ils sont nombreux à avoir tort qu’ils ont raison. -Bernard Werber
(Just because everyone else is wrong doesn't make them right)

(I wrote a short paragraph about musn't vs mustn't half a year ago.)