Can't squeeze blood from a turnip?

That's just one of those old sayings that have been around since the beginning of time. It means that no matter what you want or how hard you try it ain't gonna happen.

One of the great things about the internet is that you can google questions like this and get an instant answer!

http://www.usingenglish.com/reference/idioms/squeeze+blood+out+of+a+turnip.html

You can't extract what isn't there to begin with. Essentially meaning you can't get money from a person who doesn't have it, no matter how hard you try. Another common phrase: You can't ring water out of a stone.

i heard it first during this film and been usin' it ever since