well you see
a person once asked 250 s$p@d questions
But seriously I doubt you will find out
there are some other ones ive come accross like shut your crow mouth.
but where it comes from is anybodys guess.
best of luck

I think it is along the lines of "not the full quid" "not playing with a full deck" "50 cents short of a dollar"

its a play on words 250 sounds like you are stupid

Um where? I've never heard this anywhere in China. Must be a local thing from where you all are at. Er bei wu shi doesn't sound like any term for stupid I know. Neither does "er wu ling".

It's got to be some local slang.

It is pretty widespread in Nth China and used between the younger generation

From a post on the echinacites site:
Strangely, some numbers can even be insulting in Chinese. Being called 250 or er bai wu 二百五 means they think you're a fool, and calling a woman a 3-8, or san ba 三八, is like saying she's a witch, only spelled with a 'b'.

The stories behind these insults are a little obscure—one story has it that 250 is half of the ancient standard measure of 500 and so it's the equivalent of saying someone is half baked or not playing with a full deck. Another story is that an emperor was offering a reward as a trick to catch some assassins but instead of one killer, four turned up. He couldn't prove if any of them were lying or not so he gave them each a quarter of the 1000-liang gold reward and then had them summarily beheaded. Calling a woman a 3-8 is a much more modern insult. It is said to refer to International Women's Day, held on March 8th every year. So it's accusing a woman of being an uppity feminist.