I just happened to come across this idiom while reading a book a few hours ago; I've looked on some dictionaries but each one gives a varying definition of this idiom.
Take The Free Dictionary, for example:
"To do something that is very hard to accomplish, especially when dealing with other people in some way. Good luck getting a group of toddlers to sit still—it's like getting blood out of a stone."
Cambridge Dictionary, on the other hand, defines the idiom as:
to make someone give or tell you something, when it is extremely difficult because of the character or mood of the person or organization you are dealing with
What's with the supposed inconsistency with these dictionary entries? I still have a hard time trying to understand the true meaning of this idiom.