I think even many Chinese people cann't answer this question.

Well...It's quite difficult to translate. Hope you could read chinese. : ) http://zh.wikipedia.org/wiki/250

Alice Jim, 谢谢！

It came from a interesting story in about B.C. 300. A famous lobbyist named Su Qin was killed, and the king was very angry at it and swear to catch the murder. But he has no clue about who killed him. So he got a idea to push the murder appear by himself. He cut down the head from body and hang out of the door of Chinese castle. At the same time, posted a reward paper on the wall in centre street, which was written as follow: Su Qin is a Trojan Horse, so the peason killed him was worth to get the reward of 1000 gold. As soon as this news release out, 4 person declaim to be that so-call hero for this fortun. King laughed and say: Cut those four 250s' head! From that time on, 250 means silly and lack of brain. 　一说来源于战国故事。苏秦是战国时的一个说客，他身佩六国相印，一时很是威风，但也结下了很多仇人。后来，他终于在齐国被人杀了，齐王很恼怒，要为苏秦报仇。可一时拿不到凶手，于是，他想了一条计策，让人把苏泰的头从尸体上割下来，悬挂在城门上，旁边贴着一道榜文说：“苏秦是个内奸，杀了他黄金千两，望来领赏。”榜文一贴出，就有四个人声称是自己杀了苏秦。齐王说：“这可不许冒充呀！”四个人又都咬定说自己干的。齐王说：“一千两黄金，你们四个人各分得多少？”四个齐声回答：“一人二百五。”齐王拍案大怒道：“来人，把这四个‘二百五’推出去斩了！”“二百五”一词就这样流传下来。

Yes. just mean silly