Q From Malcolm Pack: Presuming that shan’t is an abbreviation of shall not, why do we not write sha’n’t?
A The simple answer is that people quite often used to do so. There are examples in the Oxford English Dictionary from the latter part of the eighteenth century through to the early twentieth. However, it seems it has always been more common to write shan’t than sha’n’t — the OED has 123 examples of the former against only 21 of the latter.
An early one is from a work by Fanny Burney in 1796: “He’ll make himself so spruce, he says, we sha’n’t know him again”. A late one I’ve turned up is dated 1902, from The Grand Babylon Hotel by Arnold Bennett: “We sha’n’t sell again, Prince, until we are tired of our bargain”. This spelling is now almost never seen.
What is a little peculiar is that shan’t is actually older, being found in works from the end of the seventeenth century on, as in this one from Colley Cibber’s Careless Husband of 1704: “Nay, you shan’t stir a step”. Many well-known authors preferred it, like Jane Austen, who wrote in Sense and Sensibility in 1797: “You shan’t talk me out of my satisfaction”.
I would guess that the sha’n’t form eventually lost out because the double apostrophe was a nuisance to write and looked odd. The abbreviation, as you say, strictly demands the extra apostrophe, and it was probably the influence of logically minded eighteenth-century grammarians who persuaded many people to put the extra one in to start with — but whenever did logic ultimately matter in language?