As a Mandarin slang, 250 (or 二百五 pinyin er4 bai2 wu3) refers to someone who is mentally retarded, stupid or a failure with no useful skills. The slang is based on another slang ban diao zi (半弔子 [ban4diao4zi0]) which means something similar but to a much lesser extent. In ancient China, a bunch of 1000 copper coins tied together through the square holes in the center is called a diao (弔). Ban diao zi literally means half a diao (500 coins) which refers to a person who is inadequate in skills or mental abilities. Moderate Chinese scholars often call themselves ban diao zi in their own expertise areas as a humble gesture. Therefore ban diao zi is not necessarily a pejorative term depending on context of usage. On the contrary, Er bai wu (250) is half of a ban diao zi and it is meant as an insult. (source: Wikipedia)