I think there are enough explanations here, so I'm not going to explain it again. I'm gonna tell you how to use it! You can address your close friend 二百五，it just like a joke between you and your friend! But pay attention, do never ever address a people who you're not familiar or a stranger 二百五，it just like some kind of foul language! ^^ Have fun!

yeah，idiot. The storry is very long... Long long ago (more than 2000years), a famous officer was killed. So the emperor was very angry and but no one could catch the killer. The emperor thought out a method. He let his guys flogged the corpse of that famous officer and released the information that the person who killed the officer would get 1000 两 （1 两=50g）gold. Then 4 people all said they were the murderer. So everyone could get 250 两 gold. But of course they can't got gold from the emperor，they were killed later. I am the story teller teacher, lol, so tired....

it is a long sorty about 二百五. In the acient Zhan dynasty, Qin was the strongest country, and always invaded other countries. One famous politician called Su Qin persuaded other countries unite to fight Qin,but he was killed by a murder from QIn. The king of Qi(belong to the union) was so angery, he thought out a methord to find the murder. So he announced: Su Qin is a spy, I also want him die, now somebody killed him, and removed the evil, so I want to give the person one thousand liang (A unit of weight)gold as rewards. Then four people wanted the money , they didn't know it was a strategy of the king, and they cheated to the king , said Su Qin was killed by them The king was angery but he still asked:there are one thousand liang (A unit of weight)gold, how can I give to four people . One cheater said: divided by four, 250 per people.at last the four person were killed by the king, and the real murder still alive. so today we use 250 as idiot.

500文为一封，所以250文钱就是半封。 copper coin was current money in Qing Dynasty. 500 copper coins called one "feng", so 250 copper coins call half "feng". "feng" is a measure word. sound like "crazy".so half feng=half crazy

日常生活中，人们常把傻瓜或说话不正经、办事不认真、处事随便、好出洋相的人叫做“二百五”。 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。