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 vulgar. It is also used 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., p. 193.
He might have added hisn, as in the famous distich:
Him as prigs vot isn't hisn,
Ven he's cotch'd 'il go to pris'n.