I don't know much about its origins beyond the fact that it's a contraction of 你們 nǐmen, clearly paralleled by 俺 ǎn from 我們 wǒmen and 怹 tān from 他們 tāmen (neither of which have an equivalent in most European languages). 俺 is only used as a dialectal form of 我, but 怹 is still a polite pronoun, albeit even more limited in use than 您.