From Cao Xueqin's Qing novel 《
红楼梦》 (chapter 75): a character scolds the young masters, "
只不过这会子输了几两银子,
你们就这样三六九等儿的了" — just because they lost a few taels of silver at gambling, they now divide everyone into grades. The numbers 3-6-9 evoke "many uneven ranks."