I guess you mean "十三点". This is widely uesd in Shanghai dialect. There are several different versions about the origin: 第一种说法：来源于扑克游戏 一种扑克游戏叫“抓呆子”：逐张要牌，到认为足够为止，以所要的牌点数总数相比。各牌以数字为点数，J、Q、K均作半点，A作1点，以十二点半为最大。超过十二点半，即十三点及以上，为“呆子”，最小。 因此，十三点就是“呆子”。 第二种说法：来源于牌九 牌九中有“幺五”、“幺六”两张牌。这两张牌都是“短对”，碰在一起不配对，暗指人的言行“不对”。六和七加起来是十三，还可成为一句歇后语，如“他这个人有点幺五么六”，故意把“十三点”这个不入耳的话“歇”去。 第三种说法：来源于敲钟 “十三点”初时意谓“敲乱钟”，原来旧时的时钟，每逢一小时便报时一次，例如下午4时，钟摆便会敲响4下，5点敲5下，至多为12点，也不过敲12下，但居然敲出13响，表示出现不正常情况了，正如俗语所讲“癫癫地”！所以，“十三点”就表示某人不正常、疯癫。 第四种说法：来源于“梁山伯” 《梁山伯与祝英台》中“十八相送”一折戏中，祝向梁反复借景喻情，暗送秋波，几次三番向梁表露求爱之情，可梁竟傻乎乎的无动于衷。这是剧作者的艺术手法，故意为后来的悲剧所作的铺垫和伏笔。上海人口中的梁山伯与“两三八”谐音，这三个数相加是13，讥讽梁山伯是“十三点”、“侬真是梁山伯，伊主动对侬有意思，侬还格能戆搭搭！” 第五种说法：来源于“痴”字笔画 汉字“痴”字共13画，故上海人以“十三点”隐指“痴”。以后又派生出许多与“十三点”完全同义的词，如“B拆开”、“电话听筒”（旧电话听筒中设13个小孔）等。 from 互动百科 From poker card game/clocks: normal ones could only strike twelve times /The character means Silly in Chinese has thirteen strokes. I'm sorry i cant explain it exactly. I have a long way to go with my English.

haha,I think I maybe know the answer,well,13 means B ,two parts of B.but we can't use it dirctly,because it will be shielded.you know,B means something negative.

people who live in the northern part of China don't care about 13, we don't use 13 to say the silly thing either. and i thought western people(compare with China) do avoid number 13, for they believe 13 bring bad luck to them , propably from Bible story

it do be used in shanghai.13 has the same meaning as 250,but 250 is used everywhere in china.they will mean that somebody is an idoit and ignorant.

it's used widely in Shanghai，the origin is complex。one of that is ：Chinese character’s strokes 痴chī=silly are 13。by the way ,i remember that my college teacher has told us, because a turtle has 13 scales on his back,so when u call somebody 13点，u reproved him as a turtle which was not good in chinese.