My friend Jason brought this up at the chit-chat at Xindanwei yesterday, which he in turn heard from someone else. It seems the insult 十三点, common in Mandarin, is originally from Shanghainese. In Shanghainese it’s said zəˀ sɛ ti, pinyin “se sei di”. Sounds a lot like English “society,” which, as Jason brought up, is no accident. From some BSS somewhere:

Society ，由這個詞演变而来。開埠之初的上海，傳統的上海女人是看不慣那些在交際界（society) 混的女人。洋泾浜英語把這些女人混迹的地方稱為“society”。十三點由此也就慢慢地變成了罵女人的專用詞。往後，上海人就漸漸地淡忘了十三點的本來意思，會把十三與點分開，簡化地罵：“十三伐啦？”幹脆省略去了“點”。在今天，十三作為一個專門人的名詞，已經遠遠離開了它的原來的本意。罵誰都可以用“十三點”。

Long story short, in English, like in Mandarin, calling a woman a “society” girl was a way of calling them a prostitute. This carried over into the speech of the Shanghainese during the great foreign adventurer infestation of the 30s. 十三点 was just a convenient way of writing it in the Shanghai dialect. Eventually the original meaning was lost, though not the insulting nature. Now it’s common in all Wu dialects, and can be found in Mandarin as well, though certainly less frequently.