Where did the word "yourn" originate?

OED says it's modelled on mine, so I believe it comes from the following analogy:

my is to mine as your is to yourn

The more common word yours comes from the following constructions:

her is to hers as your is to yours.

our is to ours as your is to yours.

John Bartlett, Dictionary of Americanisms (1848) offers the following entry for yourn:

YOURN. This is a contraction of your own, or a change in the termination of the pronoun yours, in conformity with mine, and which is much used by the illiterate and the vulgar. It is also used in in London, and in the West of England. "The cockney," says Mr. Pegge, "considers such words as our own and your own as pronouns possessive a little too much expanded ; and therefore thinks it proper to curtail them, and to compress them into the words ourn and yourn, for common daily use." —Anecdotes of the English Lang[uage] [1814], p. 193.

He might have added hisn, as in the famous distich:

Him as prigs vot isn't hisn

Ven he's cotch'd 'll go to pris'n.

Samuel Pegge himself says, in Anecdotes of the English Language:

Ourn and yourn are also actual Saxon Pronouns Possessive ; for the Saxon ure (our) in the Nominative Case has for its Accusative urne ; and the Saxon Pronoun eower (your) gives the Accusative eowerne ; and nothing is necessary to warrant the use of them, but a mutation of Case. Whether urne be a Dissyllable, and eowerne a Trisyllable, matters not ; because, by removing the final e (a letter of no weight in that situation), these Saxon words must ultimately terminate in the letter n, a circumstance which would soon be brought about by raid pronunciation.

Yourn, ourn, hisn, hern, theirn all originate in the Anglo-Saxon Mid Anglian dialectal grammar (Cambs, Hunts, Beds) and contain the pronoun weak ending still present in mine and thine. My family originated in Cambs and evidently retained its dialect speech even when the Industrial Revolution obliged them to move to London in the 1830s. My grandfather said yourn, etc and my father said it (and I do!) whereas my grandmother's family and my mother's family came from different parts of Suffolk (East Anglian dialect) where they didn't say it.

They are not contractions of your one or your own at all. The Samuel Pegge quotation is correct - the ancient grammar lingered longer in Mid Anglia than elsewhere which is why it has survived. The Americans and Australians inherited it through colonization.

Living in Nottingham in the 1940's and 1950's we habitually used yourn, theirn and ourn as possessives. We also said yoursen, mesen hissen for yourself, myself and himself. Despite this, my pompous grammar school English teacher insisted there was no Nottingham dialect, only laziness! Personally I feel proud at playing a part in preserving those anglo saxon forms in the unwritten common speech through hundreds of years. Sadly they now seem to be dying out thanks to the constant exposure to Mid-atlantic English via the media.