I do not think that word means what you think it means.

Just seen on the license plate frame of a Volvo, of all things, "I'm a bitch, I'm just not your's."

Now, even if "your's" was a word, it would mean, "your is," which is still just as incorrect as the contractive form. And this wasn't an email with an accidental apostrophe, it was an object whose production required foresight and special ordering, but apparently no proofreader. The driver of that car certainly isn't the English language's bitch.