Where did the slang meaning for 二百五, or érbǎiwǔ come from?
二百五 literally means 250, so how did it end up also meaning stupid or crazy? There are actually two possible explanations, and both include money!
In Ancient China, 500 taels of silver equaled 一封 (yi1 feng1: a unit of measure for silver), or one feng. 250 taels of silver, or half of a feng, is 半封 (ban4 feng1), which is a synonym for 半疯 (ban4 feng1: half-crazy). Thus, people started using the word 250 to call people crazy!
Another explanation: In ancient China, copper coins had holes in their centers. People used string in order to better carry the coins. 1000 coins on a string is called 一吊子(yi1 diao4 zi). According to Wikipedia), ancient scholars would call themselves 半吊子 (ban4 diao4 zi), which refers to someone lacking in skill or mental abilities, in order to sound more modest. 1/2 of that is 250.