you can't get blood from a turnip

proverb It is impossible to obtain something from someone if they are too parsimonious, uncharitable, or resolved against it. I've tried everything to get my toddler to eat vegetables, but you can't get blood from a turnip. I give up. Good luck trying to get Dad to give you the money. You can't get blood from a turnip. If you think I'll give you the information you're looking for, you're sorely mistaken! You can't get blood from a turnip!

blood from a stone/turnip, one can't get

This is a hopeless source of help (money, comfort, and so forth). Both stone and turnip date from the nineteenth century, and other versions exist in numerous languages. Dickens used the stone analogy a number of times, in David Copperfield, Our Mutual Friend, and other works, and health-food trends notwithstanding, it is more common today than turnip. However, Clive Cussler had the latter in Sahara (1992): “‘You can’t squeeze blood out of a turnip,’ said Giordino. ‘It’s a miracle we made it this far.’”